The Morrison government wants to introduce a uniform four-year waiting period for migrants to access welfare, in a policy expected to save the federal budget some $670 million over the forward estimates.

The government says the changes will further encourage prospective permanent residents to seek work and improve the "sustainability" of the welfare system.

It is also designed to simplify a system which has varying waiting periods for different types of payments.

The move has triggered alarm among migrant and social services advocates, who will voice their concerns before a parliamentary inquiry into the proposed changes on Monday.

The groups are particularly concerned about the implications for migrant women and children, given the types of payments which would be subject to longer wait times.

Under the new regime, which would start from January next year, pending the passage of legislation through parliament, the wait time to access carer payment, parental leave pay and dad and partner pay would be extended from two to four years.

New permanent residents would need to wait four years rather than one to access a carer allowance and family tax benefit.

Sandra Elhelw Wright is the chief executive of Canberra-based Settlement Council of Australia, a peak body for refugee and migrant services.

Speaking to The Canberra Times ahead Monday's inquiry hearing, Ms Elhelw Wright said she feared more children would grow up in poverty while their parents waited to access income support.

"Migrants, like everyone else, can come across circumstances that unexpectedly mean they need to access income support," she said.

"Nobody comes to Australia intending not to work, and nobody comes to Australia intending to access income support. But just like everyone else, they can suddenly become ill or lose their job, there can be family breakdown. All sorts of things can happen."

Ms Elhelw Wright said demand on migrant and refugee services had surged during the COVID-19 pandemic, as families unable to access income support sought emergency help. But she said food vouchers and other one-off supports were no substitute for a stable income.

She said the government should shorten, rather than extend, the waiting periods for economic and social reasons.

"We want migrants to come here and feel they are being treated equally," she said.

"That is part of what has made our multiculturalism so successful in Australia and we should be wary of anything that threatens that success."

A spokeswoman for Social Services Minister Anne Ruston said permanent migrants were expected to be "self-sufficient", typically through employment or family assistance, when they settled in Australia.

The spokeswoman said the changes would only affect future permanent residents. Refugees who arrived on humanitarian visa would be exempt, she said.

"This change ensures consistent expectations for migrants across most welfare payments, and creates a clearer and easier to understand system," the spokeswoman said.

"It further encourages self-sufficiency for newly arrived residents and improves the sustainability of the welfare system."

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