The Abbott government will save $177.3 million over four years by cutting an additional Family Tax Benefit payment that goes to big families.

Big families at present qualify for a supplement for their fourth and subsequent children, but from July 1, 2016, will lose $321 a year for their fourth child and every subsequent child.

It is understood the government will justify the saving because there are economies of scale in larger families. However, families will continue to get a per child rate for each eligible child, as part of their regular family benefits.

The supplement chop comes as families who travel overseas also take a big hit to their family tax payments.

A further $42 million over five years will be saved by restricting the time the means-tested Family Tax Benefit Part A - the main payment for families with dependent children - will be paid to families outside Australia.

At present those who are overseas can still receive their usual rate of payment for six weeks and then the base rate for 50 weeks. From January 2016, families will only get the payment for six weeks in a 12-month period while they are outside Australia. It is estimated about 35,000 families will be hit by the measure.

But families also stand to gain from the budget, with the government's previously announced $3.5 billion injection into the childcare system.

This will result in payments weighted towards lower and middle income families, with those earning about $65,000 or less receiving a subsidy that covers 85 per cent of their childcare fees - up to an hourly fee cap. The subsidy drops off to 50 per cent once families earn about $170,000 or more.

The Abbott government has previously linked the extra families payments to other cuts to Family Tax Benefits, announced in last year's budget - but these are stuck in the Senate with no sign that they will earn the support they need to pass in 2015.

The scope of the long-awaited families package was confirmed as containing four main elements on Tuesday. This includes combining the current multiple payments into one, an $869 million package for disadvantaged and vulnerable children, a $246 million trial for nannies and a crack down on families who do not immunise their children.