It was "crystal clear" the US-Israeli strikes were an illegal, armed aggression against Iran, United Nations Special Rapporteur on Human Rights and Counter Terrorism Ben Saul said.
"Every death in Iran is a violation of the human right to life as well under international human rights law," he told AAP.
"These aren't acts in self-defence because Iran has not attacked either country and is not about to imminently attack them and the security council hasn't given any authorisation."
Professor Saul accused the federal government of "trashing" rules of world order while dodging questions around the legality of the attacks.
Australia was one of the fist countries to back the strikes by US and Israeli forces on Iran at the weekend.
Tehran has retaliated with a barrage of drones and missiles aimed at neighbouring states targeting oil and natural gas infrastructure.
Asked if the strikes were legal, Prime Minister Anthony Albanese, Foreign Minister Penny Wong and Defence Minister Richard Marles each have said that was a question for the United States and Israel.
"It is for the US and Israel to explain the legal basis, and they've made comments about that," Senator Wong told reporters in Canberra on Tuesday.
Mr Albanese has said Iran's retaliation since the initial strikes showed why it was a threat to peace and stability in the region.
About 115,000 Australians are believed to be stranded in the Middle East as the conflict continues to disrupt air travel.
Australia's embassy in Saudi Arabia has warned citizens in Riyadh, Jeddah and Dhahran to shelter in place, while an Australian defence base in the United Arab Emirates was hit in an Iranian attack.
Some limited flights are resuming out of the UAE but services have been largely grounded, throwing plans into chaos and threatening to leave Australians stranded for weeks.
Prof Saul rejected arguments the strikes could be justified as pre-emptive self-defence aimed at preventing Iran from developing or using nuclear weapons.
"Iran doesn't have a nuclear weapon, hasn't taken the decision to build one and hasn't ordered that one is about to be used," Prof Saul said.
Middle powers like Australia must stand up for international law and follow suite with other countries including Spain and Switzerland that strongly protested against the attacks, Prof Saul said.
He said it was disappointing Australia seemed to support a "violation" of the United Nations Charter by "trashing the most fundamental rule of world order and peace of the last 80 years".
The UN expert suggested the federal government's cautious tone might reflect a desire to preserve the AUKUS submarine deal and avoid trade retaliation from Washington.
He argued such a stance risked emboldening countries such as the US and Israel to "keep violating international law in other situations".
Opposition industry spokesman Andrew Hastie, an Afghanistan veteran, said US President Donald Trump's four to five-week timeline for action in Iran was optimistic.
"I think the rules-based global order is dead and buried and so these sorts of legal arguments are nice, but we live in the world of reality," he told Sky News.
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