Australia will begin releasing its stockpile of fuel after the government directed fuel companies to release nearly a fifth of reserve petrol and diesel supplies.

The energy minister, Chris Bowen, acknowledged fuel supplies could face further pressure but ruled out a cut to the fuel excise or rationing fuel purchases.

Bowen said he had cut fuel companies’ minimum stock obligations to about 700m litres of petrol and 2.2bn litres of diesel respectively, freeing up about 300m and 500m respectively to be directed towards regional Australia.

Australia held about 36 days of petrol and 32 days of diesel in reserve on 3 March. Each importer and refinery will now be able to release about five days’ worth of petrol and six days of diesel stockholding.

The fuel would not be released immediately and Bowen said he could not be sure when it would reach regional Australia, where some places no longer have access to fuel, “because it’s a big country and every town is different”.

Australia’s release will count towards the world’s largest-ever coordinated release of oil reserves, ordered by the International Energy Agency on Wednesday.

Bowen said petrol usage had not changed but demand for petrol and diesel had doubled across Australia. He acknowledged a prolonged US war on Iran could put more pressure on petrol suppliers but said Australia was “nowhere near” running out of fuel.

“Will there be further threats to fuel suppliers if the international circumstance continues to worsen? Of course,” he said.

Fuel supplies had continued to arrive in Australia and 80% of the reserves, or the minimum stock obligation, remained in place, he said.

The federal government on Thursday announced it would allow lower-quality petrol with more sulfur to be used on Australian roads but did not change the standards for diesel.

The opposition’s energy spokesperson, Dan Tehan, called for Bowen to consider lower fuel standards for diesel.

“He did it for petrol,” Tehan said. “Should he do the same for diesel? Australia’s economy runs on diesel.”

The National Farmers’ Federation and One Nation have called for the government to ration fuel supply in the cities to ensure regional areas would continue to have access.

Bowen ruled out adjusting the fuel excise and said the government was not contemplating fuel rationing, though there were federal and state powers to do so.

“What we’re doing is to try to increase supply for everyone,” he said.

Andrew McKellar, chief executive of the Australian Chamber of Commerce and Industry, said industry groups were “concerned” but “not panicking” about the fuel issue, but raised concerns about future impacts of a prolonged oil supply issue.

A longer-term fuel issue, with associated price rises, would feed into inflation and higher consumer prices, he told 5AA radio.

“We can ride this out for a period,” McKellar said.

“But if we face a prolonged conflict in the Middle East that goes beyond another three or four weeks, then the pressures in the global economy will be significant and will have a big flow on here in Australia.”