No tax date this time — just the Middle East. As of September 30, 2026, Unleaded 91 across the seven Costco stations that post it averaged $2.234 a litre — up 32.7¢, or 17.1%, from $1.907 on September 1. E10, the 91-RON grade at the other seven warehouses (NSW and Queensland), rose even faster: 36.0¢ (+19.2%) to $2.238. Diesel, at all 14 stations, climbed 32.4¢ (+13.5%) to $2.731, after peaking at $2.754 on September 23. Every tracked pump got more expensive, for the second month running — the August report was about the excise cut ending; September's rise is almost three times bigger and has nothing to do with tax. Compare the month in the U.S., Canada, and the UK.
| Grade | September 1 | September 30 | Month change |
|---|---|---|---|
| Unleaded 91 | $1.907 | $2.234 | ▲ 32.7¢ (17.1%) |
| E10 | $1.878 | $2.238 | ▲ 36.0¢ (19.2%) |
| Premium 98 | $2.148 | $2.458 | ▲ 31.0¢ (14.4%) |
| Diesel | $2.407 | $2.731 | ▲ 32.4¢ (13.5%) |
National averages of posted prices at tracked Costco fuel stations in Australia, $/L. Unleaded 91 is posted at 7 stations; E10 at the other 7 (NSW and Queensland). Snapshot of calendar September 2026 — this article is not updated.
Ten quiet days, then a 30¢ climb
For the first ten days nothing much happened: Unleaded 91 drifted from $1.907 to $1.950 and diesel actually dipped to $2.400 on September 8. Then the floor gave way. Unleaded 91 jumped 8¢ overnight to $2.030 on September 12, kept stepping up through mid-month, and reached $2.244 on September 22 — 29¢ in eleven days. Diesel moved later but harder, from $2.424 on September 11 to a peak of $2.754 on September 23, with an 11¢ jump across the September 17–19 window alone. The last week was a plateau: both grades eased a cent or two and closed at $2.234 and $2.731.
Every state rose; the ACT and NSW led
Ranked on 91-RON — Unleaded 91 where the warehouse posts it, E10 in NSW and Queensland — Canberra added 38¢ (+19.5%), the biggest state-level move, and New South Wales was right behind at 37.5¢ (+20.0% on a lower base). Lake Macquarie's E10 rose 46¢, the largest station move on the network, after being the month's laggard in August. Queensland and South Australia each rose 34¢ — all three Queensland warehouses (Gold Coast, Ipswich, and North Lakes) moved by exactly the same amount — and Western Australia rose 32¢ at both Perth Airport and Casuarina. Victoria rose the least, 31¢, because Epping added only 28¢, the smallest rise of any station. Even the smallest rise was close to 30¢.
| State | Stations | 91-RON now | Month change |
|---|---|---|---|
| ACTbiggest riser: Canberra | 1 | $2.327 | ▲ 38.0¢ (19.5%) |
| New South Walesbiggest riser: Lake Macquarie | 4 | $2.247 | ▲ 37.5¢ (20.0%) |
| Queensland | 3 | $2.227 | ▲ 34.0¢ (18.0%) |
| South Australiabiggest riser: Adelaide | 1 | $2.227 | ▲ 34.0¢ (18.0%) |
| Western Australia | 2 | $2.187 | ▲ 32.0¢ (17.1%) |
| Victoriabiggest riser: Moorabbin | 3 | $2.237 | ▲ 31.0¢ (16.1%) |
September movers by state-average 91-RON at Costco (Unleaded 91 or E10), September 1 – 30, 2026, ordered by the size of the rise. Every state rose; in Queensland and Western Australia every station rose by the same amount.
Every state, ranked by September 30's Costco 91-RON
The spread across the six states and territories we track widened to 14.0¢ a litre— Western Australia's $2.187 against the ACT's $2.327, up from 8.7¢ at the end of August. Perth Airport and Casuarina at $2.187 are the cheapest 91-RON fills in the country, followed by Epping at $2.207. Canberra at $2.327 is the dearest, ahead of Lake Macquarie's E10 at $2.287 and Moorabbin's Unleaded 91 at $2.277. Most of the rest of the network — Ardeer, Auburn, Adelaide, and all three Queensland warehouses — sits at exactly $2.227.
| State | Stations | Costco 91-RON |
|---|---|---|
| Western Australia | 2 | $2.187 |
| Queensland | 3 | $2.227 |
| South Australia | 1 | $2.227 |
| Victoria | 3 | $2.237 |
| New South Wales | 4 | $2.247 |
| ACT | 1 | $2.327 |
State-average 91-RON (Unleaded 91 or E10) across tracked Costco fuel stations, September 30, 2026.
Why prices climbed in mid-September
This was a wholesale move, not a tax one. The NRMA tied the mid-month turn to the US striking Iranian military targets and deploying mines in the Strait of Hormuz, and the ACCC said retail prices were influenced by high international refined fuel benchmark prices as the conflict continued. The national figures published by the Australian Institute of Petroleum, as reported by the ABC, trace the same shape as Costco's chart: unleaded averaged 205.5¢ in the week to September 6, 211.1¢ to September 13, and 227.3¢ to September 20, with diesel going from 252.9¢ to 273.6¢ over the same span.
By the week to September 23 the ACCC's five-city average was $2.37 for petrol and $2.87 for diesel, up 6% and 7% in a single week. Sydney regular unleaded had hit 237.8¢, up 22.3¢ in a week, with Sydney wholesale unleaded up 14.8¢ and wholesale diesel up 23.3¢. Against those benchmarks Costco held its usual discount: its $2.247 Unleaded 91 and $2.754 diesel on September 23 were both about 12¢ under the five-city averages for that week.
Up 43¢ since the excise cut ended
Costco Unleaded 91 averaged $1.807 on August 1, just before the excise snap-back. Two months later it is $2.234. August added about 11¢ as the tax came back; September added 32.7¢ more, with no change in tax. The government has ruled out another immediate reduction in fuel excise, so there is no policy cushion coming.
What to expect in October
The plateau in the last week of September matters. The ABC reported on September 23 that Brent had dropped from roughly US$110 to about US$98 in a week, and the NRMA said Singapore gasoil, the diesel benchmark, had eased from US$200 to US$174. But the RACQ said it did not expect an immediate drop, only “potential for fuel prices to decrease by up to 5 cents per litre” if lower global prices hold, and Swinburne's Hussein Dia pointed to fuel already in the supply chain bought at higher prices. The downside risk is real too: AMP's Shane Oliver warned that a failure to resolve the crisis could send oil to US$150 a barrel and petrol above $2.70 a litre. The city price cycles will still swing on top of whatever wholesale does, so a mid-cycle dip at Costco is not the same as a turn. Perth Airport and Casuarina at $2.187 remain the cheapest Costco 91-RON in the country.
Costco pump prices are a snapshot of calendar September 2026 (September 1–30) and will differ at the pump today. This site is independent and not affiliated with Costco Wholesale Corporation.
