A small number of petrol retailers are being investigated by the consumer watchdog for not passing on a 20-cent cut to fuel excise, despite the tax cut being passed on to motorists in the “vast majority of locations”.

With petrol prices topping $2 a litre in most capital cities, the government has left open the possibility of extending the 50% excise cut beyond its 28 September expiry date, despite the treasurer, Jim Chalmers, saying it will “probably” end as planned.

The Australian Competition and Consumer Commission, which was tasked with monitoring the government cut to the fuel excise announced in the March budget, said recent price hikes had followed an increase in international petrol prices.

From mid April to early July, regular unleaded petrol wholesale prices increased by around 40c a litre and diesel wholesale prices increased by around 42c a litre, with this flowing through to retail prices.

An ACCC spokesperson said its petrol market monitoring showed that “the cut to the fuel excise has been passed on to Australian motorists in the vast majority of locations”.

“In the very small number of locations where our analysis showed average prices didn’t decrease as much as the cut in excise, we have sought more information from the petrol retailers and are continuing to look into it,” the spokesperson said.

“Retail petrol prices constantly fluctuate with changes in international prices and the price cycles in the largest capital cities, even though the excise cut has flowed through to the bowser.”

The ACCC would not release information about the number of complaints received from motorists concerned the excise cut may not have been passed on, and would not disclose which matters were under investigation.

The popular budget measure, put in place as a temporary cost-of-living relief measure in the lead-up to the election, halved the fuel excise from 44.2c to 22.1c a litre at a cost to taxpayers of about $3bn over six months.

In a Guardian Australia podcast earlier this month, Chalmers said that “most probably” the tax relief would end in September.

“It would be incredibly hard to be able to afford to continue some of this cost-of-living relief, this specific relief, indefinitely,” he said.

“Obviously, we factor in the conditions as they evolve, and the budget and all of the rest of it. But nothing has substantially changed to make me think that we could continue that indefinitely, or even for a substantially longer period than September. So that’s the expectation.”

The opposition’s finance spokeswoman, Jane Hume, said through a spokeswoman that extending the fuel excise cut was “a matter for the government”.

“The Coalition cut the fuel excise to provide temporary and targeted cost-of-living relief for Australians,” she said.

“At the time Treasury advice was that this would both assist with cost of living pressures, while not contributing to inflation.

“The Labor government was elected on a promise to bring down the cost of living but it’s yet to offer any solutions. They need to outline an economic plan that addresses costs of living for families.”

Figures from the Australian Institute of Petroleum show the average retail petrol price for the week ending 10 July was 212.1c while the average wholesale price was 196.9c. Prices were highest in Melbourne and Canberra, and lowest in Perth and Brisbane.

Extracted in full form: Small number of Australian petrol stations investigated for not passing on fuel excise price cut | Australia news | The Guardian