US submarines carrying nuclear weapons could dock at Australian bases, defence officials have told the Senate, and the Australian government and people would not know.

Senate estimates heard fierce debate over whether US Virginia-class submarines – set to “rotate” through Australian ports from 2027 as part of the contentious Aukus agreement – could carry nuclear weapons. Nuclear weapons are prohibited in Australia.

Defence department officials told senators there was “no impediment” to submarines armed with nuclear weapons visiting Australia, insisting that any such visit would not breach Australian or international law.

The US maintains a policy of “strategic ambiguity” around its nuclear deployment and it refuses to confirm or deny whether aircraft or seagoing vessels capable of carrying nuclear weapons are, in fact, carrying a nuclear warhead.

That ambiguity would apply to US submarines that might dock at Australian ports, as it now does to nuclear-capable B-52 bomber aircraft landing at RAAF Base Tindal in the Northern Territory, which is being upgraded to be able to accommodate more US bombers.

“We respect the United States position of neither confirming nor denying,” the Australian defence department secretary, Greg Moriarty, told the Senate.

But anti-nuclear campaigners and some senators argue that the evidence to the Senate undermines Australia’s non-proliferation commitments and risks making the country “a launchpad for nuclear war”.

They also say the acquiescence to US ambiguity over its nuclear weapons contradicts the foreign affairs minister’s assurance in a 2023 speech that only conventionally armed submarines would visit Australia.

“The US has confirmed that the nuclear-powered submarines visiting Australia on rotation will be conventionally armed,” Penny Wong told the National Press Club.

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US Virginia-class submarines are to begin rotating through Australian ports – part of pillar one of the Aukus agreement – from 2027, before Australia buys then builds its own nuclear-powered, conventionally armed submarines.

The US Congress approved funding in 2024 for a new submarine- and ship-launched nuclear missile – the first new US nuclear weapon since the end of the cold war. The SLCM-N weapon is slated to be operational within a decade and Vice-Admiral Johnny Wolfe has told Congress that the weapons program is “focused on the integration of SLCM-N into the Virginia-class submarine”.

The South Pacific nuclear free zone treaty – known as the Treaty of Rarotonga and to which Australia is a party – prohibits the “stationing” of nuclear weapons in Australia (and the broader South Pacific zone). But Australia maintains that a US submarine, potentially armed with nuclear weapons, is not prohibited from visiting an Australian port.

Bernard Philip, assistant secretary of international policy at the Department of Defence told the Senate Australia would comply with its treaty obligations, which were also understood by the US.

“The United States does not station nuclear weapons in Australia,” he said. “Stationing nuclear weapons in Australia is prohibited by the South Pacific nuclear free zone treaty, to which Australia remains committed.

“There is no impediment under the Treaty of Rarotonga and the Treaty on the non-proliferation of nuclear weapons to the visit of dual-capable foreign platforms to Australia’s territory or transiting Australia’s airspace or waters.”

Dual-capable platforms are those able to carry both conventional and nuclear weapons. Defence officials said nuclear missiles deployable on Virginia-class submarines were still in development, labelling the scenario “hypothetical”.

The Australian director of the International Campaign to Abolish Nuclear Weapons, Gem Romuld, said the foreign minister’s commitment that nuclear weapons would not be rotating through Australia was now “dead in the water”.

“It’s taken just two years for expectations of an Aukus partner to shift, so what will come next?” she said.

“If Aukus is ‘not about nuclear weapons’, then Australia’s numerous assurances must be backed up with legal commitments. The best way to draw the line on nuclear weapons is to sign and ratify the UN Treaty on the Prohibition of Nuclear Weapons.”

Labor has formally committed in its party platform to signing and ratifying the weapons ban treaty “in government” but it has not yet signed done so. Seventy-four countries are now party to the treaty: none of the nine nuclear-armed states have signed it.

The Greens senator David Shoebridge asked in Senate estimates: “So we’re going to permit them to be sitting, floating off Fremantle in US-Virginia class submarines, and is it still the position that the Australian government won’t ask the US whether or not they’re nuclear-armed submarines, just like we don’t ask about the nuclear-armed B-52s?

“Is that still the position? Don’t ask, don’t tell?”

The secretary of the defence department said: “We respect the United States’ position of neither confirming nor denying.”