Single parents will soon have a huge disincentive to earn more than $48,000 a year due to a design quirk in the Abbott government's cuts to family payments.

Analysis from the National Centre for Social and Economic Modelling shows the government's new single-parent family supplement - designed to offset the impact of the government's planned cuts to family tax benefit B - will result in prohibitively high effective marginal tax rates for single-parent families.

It means single parents may be discouraged from earning more than $48,000 a year because they will lose their supplements entirely.

Currently, single-parent families with a child aged between six and 12 receive $3500-$4000 a year from family tax benefit B. But from July 2015 they will no longer receive the payment when their youngest child turns six. Instead, they will be eligible to get a special new ''supplement'' of $750 for every child aged between six and 12.

NATSEM's analysis shows that once a family earns $1 more than that - $48,001 or more - the family will no longer be eligible to receive any supplement. That means they will in effect get only 20¢ in every dollar earned beyond $48,000, resulting in an effective marginal tax rate of about 80 per cent.

''It's called a sudden-death drop,'' NATSEM's Ben Phillips said. ''The moment you go from $48,000 to $48,001, you will lose that payment entirely.''

The analysis shows that if the income of a single parent with two children increases from $45,000 to $50,000 a year, their disposable income will increase by only $1706.