Australian motorists may have to wait weeks for the Morrison government’s halving of the fuel excise to be passed on, with one motoring group warning that retailers could use falling global oil prices as a cover to fatten their profit margins.

From midnight Wednesday, the government cut the fuel excise by 22.1 cents a litre for six months at a net cost to the budget of $3bn.

Petrol prices have been decreasing for the past fortnight in most state capitals, with falls in recent days largely unrelated to the tax cut.

Guardian Australia understands that service stations are now selling fuel that was supplied to them with the 44.2 cent a litre excise, and are only obliged to cut the bowser price when they clear those stocks and take on new supplies. Retailers with slower turnover would work through their more expensive fuel first.

Unleaded petrol in Sydney was averaging $1.91 a litre on Thursday afternoon, according to Fuel Check. Adelaide had the lowest price at $1.80 a litre, with Hobart the highest at $2.12, and Melbourne, Brisbane and Perth at about $1.90, the Fuelprice Australia website showed.

Petrol and diesel pumps at a service station in Sydney
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Peter Khoury, a senior spokesperson for NRMA, said petrol prices waxed and waned according to retailers’ discount phases. Global oil prices, which set the broader trend with a couple of weeks delay, have posted “significant” falls in recent weeks.

Gas-oil prices, which set diesel prices, peaked on 9 March and have fallen US$40 (AU$53) a barrel since, while Singapore’s Mogas 95 had dropped at least US$20 (AU$27). A stronger Australian dollar compared with the greenback should help nudge local prices lower, Khoury said.

“What we don’t want is a situation … where relative falls in oil prices are merely substituted by the cut in the fuel excise,” he said. “We absolutely want to see the falls in all prices passed on in full.”

The chair of the Australian Competition and Consumer Commission (ACCC), Gina Cass-Gottlieb, said earlier this week that the market watchdog would monitor to see if service stations did pass on the excise reduction.

Cass-Gottlieb said it could “compel refiners, importers, terminal operators, wholesalers and retailers to provide information relating to fuel prices where necessary”.

“We expect that fuel retailers will pass on the cut in fuel excise to reduce the price at the bowser as soon as possible, as existing petrol stock levels are used up,” she said.