TL;DR: Brisbane diesel retail moves with the wholesale price — the Terminal Gate Price, or TGP — not with the petrol cycle. But it moves slowly: about 13.8 days to close half a wholesale move, roughly a month for most of it. Over this window diesel retail tracks the TGP closely (level correlation 0.94 — though that level figure rides a shared long-run trend; the de-trended week-to-week read is a sharper 0.80); petrol barely tracks wholesale at all (0.13). Measured on the week-to-week average move, diesel is about five times smoother than petrol — 2.6 c/L of swing versus petrol’s 12.5. (That’s a different metric from our earlier diesel post’s monthly trough-to-peak figure of 4.5 vs 45; more on that below.) Right now the two-week lag is playing out live: a +20.5 c/L wholesale surge into 17 July has pulled the metro diesel median up to 225.9 c/L — up from the 214.9 I reported on 20 July, a move consistent with the lag.
I drive a diesel ute, so I get asked the wrong question a lot: “when’s the diesel cheap Tuesday?” There isn’t one. Diesel doesn’t run the petrol cycle. But it isn’t random either — it’s chasing something. One of you named it in a review of the other app:
“Diesel is used by industry a lot, so there’s no incentive to cycle the pricing… the consumer/retail market for diesel is a lot smaller.” — r/CarsAustralia
Right. No discount game. Instead diesel rides wholesale — and there’s a lag before a wholesale move reaches your bowser. This post is about how long that lag really is.
The claim
Diesel at the pump follows the wholesale price, not the calendar. And the “two-week lag” everyone quotes is real — but it’s a half-life, not a countdown. The pump doesn’t sit still for 14 days then jump; it starts moving almost straight away and takes about two weeks to get halfway there.
What we checked
Brisbane metro, the 473 stations within 30 km of the CBD that quote diesel, from 1 January 2025 to 23 July 2026. For each station, each day, I carried its last price change forward and took the median across the stations — the same daily-panel method our earlier diesel piece used. Then I lined the retail median up against Brisbane wholesale: the Terminal Gate Price for the Brisbane region, diesel and ULP separately, forward-filled across the weekend gaps the TGP naturally has.
Two things worth stating up front:
- Diesel means the diesel family — standard plus premium diesel — because plenty of metro stations report their diesel under the premium code. Read one code alone and you get a wrong, sparse answer.
- We ran petrol as the control. Same method, same window. If diesel tracks wholesale and petrol doesn’t, that’s the whole thesis in one comparison.
Then three lenses on the speed: a lag-scan correlating the two series; an error-correction model that measures how fast retail closes the gap to wholesale each day; and an event study of what the pump does in the 1 to 21 days after a real wholesale shock.
What the feed shows
Diesel tracks wholesale. Petrol doesn’t. On the raw price level, diesel retail and the Brisbane TGP move almost lockstep — correlation 0.94 at a two-day lag. Petrol against the same wholesale series: 0.13 — the same near-zero tracking our earlier diesel piece found. That’s the difference between a price set by cost and a price set by a discount cycle.
Diesel is about five times smoother. Week to week, the diesel median moves an average of 2.6 c/L. Petrol moves 12.5 — the sawtooth doing its thing, one week of the cycle at a time. Diesel has no sawtooth to ride. One reconciliation, because the numbers look different from our earlier diesel piece: that post measured the monthly trough-to-peak swing (diesel 4.5 vs petrol 45, about ten times); this is the week-to-week average move (diesel 2.6 vs petrol 12.5, about five times). Two different rulers on the same fuel — the monthly figure is peak minus trough across a whole month, this one is the typical size of a single week’s step — so read “five times” and “ten times” as the same story measured over different spans, not a contradiction.
The lag is a half-life, not a countdown. Retail starts chasing a wholesale move almost immediately, but it only closes about 4.9% of the remaining gap per day. Compound that same rate day after day and it takes 13.8 days to absorb half a move: about 30% through by day 7, about 40% by day 10, half a touch before day 14, and roughly 78% — most of it — by day 30. That single geometric curve is what the chart below plots. So “two weeks” is right if you read it as “about two weeks to get halfway”, not “nothing happens for two weeks then it jumps”.
One honest note on the estimate: a separate lens — an event study that watches the pump in the days right after a real wholesale shock — runs a little ahead of this curve, reading closer to 39% by day 7. That’s the difference between averaging over a clean set of shocks and fitting one adjustment rate to the whole series; I’ve plotted the error-correction curve here because it’s the consistent, whole-window model, and flagged the event-study read so the two estimators aren’t mistaken for one line.
Falls flow through faster than rises. Splitting the wholesale shocks by direction over this window, a two-week look shows down moves passing through at about 56% while up moves net roughly 0% — the rise stalls, the fall carries. Note the direction here is the opposite of the petrol grievance: the petrol up-fast-down-slow asymmetry is a retail cycle mechanic — the majors hoist fast and drift down slow on top of wholesale — whereas this is diesel tracking wholesale, where over this thin window it’s the falls that pass through and the rises that drag. Same words, different machine. Read it as a tendency over this window, not a law — it’s a thin sample and it shifts with where you set the shock threshold.
The verdict
Diesel is a wholesale-tracking fuel with a slow pump. When the TGP moves, your bowser moves too — just with a couple of weeks of drag before half of it lands, and a full month before most of it does. There’s no trough to time because there’s no cycle; there’s only the wholesale line, and how far the pump has caught up to it.
So what does that mean for your tank?
Right now, this is live. Brisbane diesel wholesale surged +20.5 c/L into 17 July, taking the TGP to 202.1 c/L. Add the typical 22.7 c/L gross margin that sits over wholesale — the 18-month median gap, made of freight, excise handling, GST and the servo’s own costs, not profit — and the “fair” pump price works out near 224.8 c/L. The live metro diesel median is 225.9 (a live gross gap of 23.8 c/L, near that long-run 22.7). So the pump has already caught up to that surge; it’s not a rise still coming, it’s a rise that’s landed.
You can watch it in this masthead’s own record. Four days ago, in the 1-August excise post, I clocked the metro diesel median at 214.9 c/L, sitting near the top of its 30-day range. Today it’s 225.9 — about 11 cents higher in four days, no cycle involved. That’s not a mystery jump; it reads as the +20.5 wholesale surge working its way through to the bowser, consistent with the lag. One live episode isn’t proof of an averaged model — it’s one read that fits it — but it’s the mechanism you’d expect. If you were waiting for it to “settle back down”, the wholesale line says don’t hold your breath — it settles when the TGP does.
That’s the honest use of the lag: it tells you whether a bowser rise is a real cost that’s here to stay, or froth. The way to check for your own fill is the spread across servos on the day — on any given day that spread is wider than a fortnight of wholesale drift. Pick diesel on fill.pinly.com.au, see the spread on the map, drive to the cheap one. Every price there is the one each station reported to the Queensland Government feed — not a stranger’s guess at the bowser.
One honest limit
The two-week figure is a half-life averaged over 18 months, not a promise about your next fill — and the up-versus-down asymmetry rests on a thin sample (66 to 103 events a side) that moves with the threshold, so treat it as a tendency, not a rule. The pass-through curve I plotted is the error-correction model’s single geometric path; the event-study lens reads a touch faster in the first week (nearer 39% by day 7), and the honest read is somewhere between the two. And the live 214.9→225.9 move I lean on above is a single episode read against that averaged model — it’s consistent with the lag, not a validated forecast; one live surge can fit the curve and still not prove it. The TGP is Brisbane-region wholesale, published near-daily with weekend gaps and its own few-days reporting lag, so don’t read it against retail second by second. And the 22.7 c/L margin is gross — an 18-month median, an anchor for “is this rise real”, never a profit figure.
Scope: Brisbane metro (473 stations within 30 km of the CBD that quote diesel), diesel family (standard + premium diesel) versus ULP as the petrol control, Queensland Government price feed, 1 January 2025 – 23 July 2026, with the 1 March – 27 April 2026 data gap excluded everywhere. Wholesale is the Brisbane Terminal Gate Price by fuel type. The pass-through chart plots the error-correction model (adjustment ~4.9%/day, half-life 13.8 days) — the geometric pass-through curve described in the chart caption; a separate event study on wholesale shocks of at least 3 c/L reads a little faster in the first week and is noted, not plotted. The 22.7 c/L diesel gross margin is the 18-month median gap of retail over wholesale (the live gap on 24 July was 23.8). Live anchor from the latest price each station reported to the Queensland Government feed on 24 July 2026: metro diesel median 225.9 c/L.