Tying HECS repayments to family income would "substantially reduce" Australia's spiralling university student debt problem, says a leading education policy expert.

Andrew Norton, the education program director at think tank the Grattan Institute said that "the radical change" of measuring HECS-HELP repayments against the income of an entire household would go to the heart of the nation's university debt crisis.

"It is the part-time workers in families who are, long term, earning below the threshold who can and should pay to alleviate the problem," he said.

The Grattan Institute's research found that nearly half the students with HECS debts and income below the current threshold of $54,000 live in households with disposable income exceeding $80,000 a year, including 30 per cent with disposable incomes over $100,000 a year. Fourteen per cent of people who would start repayments at the Grattan Institutes proposed $42,000 threshold live with their parents.