Federal Government estimates on the numbers of parents affected by the changes show that 37 per cent of all those who receive payments will get more under the new scheme but 49 per cent will get less.

Nearly half of parents who pay child support - 51 per cent - will pay more but 33 per cent will pay less.

The Government argues that parents whose payments are reduced are unlikely to be worse off because they could receive higher family benefits payments.

"The changes respond to trends towards increased shared parenting and recognise that when care of a child is shared, the costs are also shared,"the Minister for Human Services, Joe Ludwig, and the Minister for Families and Community Services, Jenny Macklin, said.

But the changes hit people receiving income support payments the hardest. The estimates show that 51 per cent of single parents receiving income support will get lower child support payments and 33 per cent will receive more.

Most of those who lose money will be getting up to $20 a week less.

The first payments under the new scheme were due to be paid yesterday and the Government has promised to monitor the effect of the changes, which were introduced by the previous government and supported by Labor in opposition.

The changes - the first since 1988 - were prompted by a parliamentary review that found the scheme was inequitable and had failed to keep pace with increased government payments to families.

As a result of the changes, payments are no longer based on a percentage of the taxable income of the payer. Instead, they are based on the parents' combined income and will recognise the time each parent spends caring for a child.

Assessments will include children from other relationships.

The changes are also designed to help the Government recover the child support debt which grew by 5.6 per cent last financial year to $1 billion.