Legislation before Parliament would protect Australian Defence Force personnel from civil and criminal liability when they assist state and territory responses to emergencies, and would remove one step in the government's procedure to deploy the military during disasters.

However two legal experts have said the bill does not identify whether it is constitutional for the defence force to provide civil aid during a disaster or emergency.

Director of the Australian National University's Centre for Military and Security Law David Letts told a Senate inquiry the proposed laws didn't address the legal basis for using the military in emergency responses.

In January he called for the federal government to remove any potential legal uncertainties in its historic call-out to military reservists.

Associate Professor Letts told the Senate inquiry into the bill last month the proposed laws raised questions about the scope of the federal government's power to use the military in response to disasters and emergencies.

He said doubt should be removed about the lawfulness of deploying the defence force to help in disasters.

"Proper understanding of the constitutional basis that authorises the use of the ADF in response to emergencies is something that would benefit both the ADF and the Australian public, so that there is no doubt that any deployment of the ADF occurs pursuant to valid lawful authority," Associate Professor Letts said.

The proposed laws failed to establish the military's authority to use force or other coercive powers in emergencies, he said.

Associate Professor Letts warned defence personnel had no specific authority to help civil authorities deal with people refusing to comply with directions to leave an area, or to conduct searches and seize property.

"It appears, fortunately, that this issue has not arisen in any significant way during either the bushfire response or the COVID-19 response but the possibility is nevertheless there," he told the Senate committee.

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University of Sydney constitutional law professor Anne Twomey said powers to call out reservists were limited because civil aid responding to natural disasters or pandemics did not fall within the scope of "defence" in the constitution.

The Defence Act didn't deal with aid to the civil community by the regular forces of the ADF during natural disasters or emergencies, because such laws would lack constitutional power.

"Yet the ADF still fulfils these activities by relying on the Commonwealth's prerogative powers, presumably in the hope that the prerogative is so murky and uncertain in its application that its exercise is harder to challenge," Professor Twomey said.

"This does, however, leave the ADF legally exposed."

Passing the new bill would mean there was no direct authority in the Defence Act for the military to provide assistance, but defence personnel would have immunity from criminal and civil liability in helping with responses to emergencies, she told senators.

"This bill is only adding to the conceptual mess, rather than fixing it," Professor Twomey said.

Under the proposed laws, defence personnel and members of foreign forces would receive similar protections to those of state and territory emergency services personnel from liability while supporting civil emergency and disaster responses.

In its final report, the bushfire royal commission last week recommended the government provide appropriate legal protections for ADF personnel.

The Governor-General would also no longer have to act on the advice of the Executive Council in making a call-out order to defence reservists, under the bill.

The Senate's Foreign Affairs, Defence and Trade Legislation Committee is expected to release its report on the bill on Wednesday.

The ADF supported bushfire responses between September 2019 and March this year, undertaking more than 1500 tasks during Operation Bushfire Assist and deploying more than 8,200 military personnel including 2,500 reservists.