ABOUT 10,000 single mothers may be left empty handed under changes to parenting payments.

From January 1, the majority of the 84,000 single parents, mostly mothers, receiving parenting payments will be moved to the Newstart allowance when their youngest child turns eight.

The federal government will save $728 million over four years as a result.

Charities were furious when the legislation passed parliament in October and believe the changes will leave some parents $140 a week worse off and increase homelessness.

Human Services Minister Kim Carr declined to specify how many parents would be worse off.

"There is clearly a substantial difference in the amounts of money that are available between the two payments so there will be a reduction for many people," he told ABC Radio on Thursday.

"The policy rationale is to encourage people to take on more work."

He expects the majority of parents affected by the changes to be shifted to Newstart.

About 3000 will be eligible for disability and carers' support payments.

Senator Carr expects that about 10,000 parents will be earning income above the allowed threshold and won't be eligible for any welfare assistance.

He said 5000 would receive part payments.

His department initially estimated that 17,000 parents would be ineligible for payments, but the latest figure was closer to 10,000.

Asked if the revision would affect anticipated budget savings, Senator Carr said the matter was for "others to calculate."

He said there would be career advice, job service support and childcare support for parents affected by the welfare cuts.

Australian Greens senator Rachel Siewert said the 10,000 parents ineligible for assistance are being "unfairly punished for doing exact thing the government says they should".

Senator Siewert said the revision of figures of those ineligible could affect expected budget savings.

"It may mean less (budget) savings for a whole lot of pain," she told AAP.

"The (federal government) is disguising the fact they are dropping single parents and their children further into poverty."

She dismissed Centrelink's efforts to place individual calls to the 84,000 parents who will be affected by the cuts.

"The government's alleged commitment to support of vulnerable people makes a mockery of the process they've undertaken on this legislation to date," Senator Siewert said.