Senator Molan, a former army major-general, said Australia had a strong national interest in ensuring the free movement of tankers because it has low liquid fuel reserves at home and therefore could be crippled by any serious global blockage.

"I believe Australia's self-interest is involved in both joining the Americans in escorting tankers in and out of the Gulf and providing surveillance for that activity as well as anything we can do in relation to sanctions," Senator Molan said.

"But for any significant military action, we should reserve our right to make a subsequent decision."

Australia typically has a navy ship in the Gulf region and can operate maritime surveillance or aerial refuelling aircraft out of bases in the Middle East, which Senator Molan said could help keep an eye on Iranian activity in international waters.

The Trump administration, which pulled out of the 2015 nuclear deal negotiated under former president Barack Obama just over a year ago, is now trying to pressure Tehran back to the negotiating table.

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Iran has said it is enriching uranium and will, by June 27, breach the 300 kilogram limit on low-grade fissile material it is permitted under the deal.

Australia is not a signatory to the deal but has expressed diplomatic support. After a review of that support last year, Prime Minister Scott Morrison said Australia would maintain its position because the deal was achieving its aim of "substantial restrictions on Iran's nuclear activity".

The Herald and The Age have asked the government whether it will continue its support if Iran breaches the 300 kilogram cap or if it enriches even more, as it has threatened to do after July 7 unless European nations help it repair some of the damage to its economy caused by US sanctions. The government had not responded by deadline on Wednesday.

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Rodger Shanahan, a Middle East expert with the Lowy Institute, said Iran was likely trying to get itself deliberately into technical breach of the deal so that it could then offer to peg itself back to the 2015 limits as a bargaining chip if and when negotiations with the US resume.

He said Tehran was hoping to outlast Middle East hawks within the Trump administration, believing that officials such as the hardline national security adviser John Bolton would likely not survive in their roles beyond next year's presidential election, assuming Mr Trump wins.

"The Iranians' patience will probably outlast Trump's attention span on this issue, which is what they're banking on," he said.