The Federal Government has conceded 1.5 million people will see a reduction in Family Tax Benefit (FTB) supplements, but argues its welfare changes will be easier for 1 million children to receive child care.

The Government has previously shied away from revealing how many people would be affected by the omnibus bill introduced into Parliament on Wednesday.

After repeated questions this morning, Social Services Minister Christian Porter said all 1.5 million families who receive FTB payments would see their annual supplements cut.

"The supplements are scaled down for all the people in the system, then there is a $20 increase (per fortnight for FTA-A recipients) and those two interact," Mr Porter told reporters at Parliament House.

He added 1 million children and their families would benefit from the changes to child care, though he was unable to say how many would take advantage of the measures.

"This is once-in-a-generation reform to child care that Australian families are absolutely screaming out for," Mr Porter said, who argued an additional 1 million children who attend after-school care could also benefit.

As part of the omnibus bill, those receiving Family Tax Benefit payments will receive an extra $20 a week, but will relinquish annual supplements, starting from July next year.

The Government has offered $4 billion in sweeteners over the next four years, in exchange for $8 billion worth of welfare savings or cuts.

The Federal Opposition confirmed it would vote against the omnibus bill.

It argues families on FTB-A would be $200 a year per child worse off, while those on FTB-B would be $350 a year worse off as a family.