The Federal Opposition says new figures from the National Centre for Social and Economic Modelling (NATSEM) show low-income families will lose the most if the budget is passed in full.

NATSEM's modelling, which was commissioned by Labor, shows a single-income family on $65,000 a year with two children would lose nearly $21,000 over four years.

A sole parent with two children on $55,000 would also be more than $20,000 worse off over four years — or up to $6,000 a year.

Labor's families spokeswoman, Jenny Macklin, said the figures showed the budget was grossly unfair and would hit hundreds of thousands of low and middle-income families.

"The message from these figures is that the only way to protect these families is to reject these cuts in the Senate when the Government next presents them," Ms Macklin told AM.

The modelling shows the lowest income families would lose about 7 per cent of their disposable income, whereas the richest families would have a tiny gain.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Listen Duration: 3 minutes 13 seconds 3 m 13 s Listen to Louise Yaxley's report Download 5.9 MB

The modelling included the Government's plan to stop Family Tax Benefit Part B when the youngest child turns six, which the Senate has refused to pass from last year's budget.

Parliamentary secretary to the prime minister Christian Porter told Sky those measures would fund a proposed increase to child care.

"The fiscal reality is that there must be savings that you garner to pay for the package," Mr Porter said.

Budget coverage in your inbox Subscribe to get ABC News email updates, including reports and analysis of the federal Budget fine print and how it will affect you, as well as alerts on major breaking news.

"These are the savings that have been put on the table and we have to negotiate around these savings to make the package happen.

"I think that might just focus the mind of crossbenchers.

"Because if they go back to their constituencies who want the benefits of the childcare package, and who I think ultimately will be willing to live with savings in other areas, that ultimately will put pressure on the crossbenchers that maybe hasn't been there before."

Ms Macklin said asking families to accept the trade-off was unfair.

"The Government should not be saying to families that face a cut to their income of more than $6,000 a year that they have to take that cut for other families to get improvements to child care," she said.

"That is just outrageously unfair and families are very, very angry that that is the way that Tony Abbott is treating them."