The Greens are offering the government a $13 billion budget saving. All it has to do is ditch its commitment not to touch superannuation before the next election.

The plan would tax superannuation contributions on a progressive scale rather than the present flat rate of 15 per cent and 30 per cent for workers earning more than $300,000.

Australians on the 19 per cent marginal tax rate would pay 4 per cent on their super contributions, Australians on the 33 per cent rate would pay 15 per cent, Australians on 37 per cent would pay 22 per cent, and Australians on the 45 per cent rate would pay 30 per cent.

As a supporting measure, the policy would also clamp down on "churning" wages through super funds. It will no longer be possible for Australians over 55 to get a tax benefit just for putting their salary into a super fund while drawing an equivalent wage from the same fund.