The budget would be almost $3 billion a year better off under a Greens plan to cut superannuation tax breaks for people earning more than $80,000 a year, the Treasury has found.

After a week of fighting over public servants' role in costing opposition policies, the Treasury  released costings of a Greens super policy, in response to a freedom of information request from the Coalition.

The overhaul proposed by the Greens would see concessional super contributions taxed at a person's marginal tax rate.

At the same time, they would also receive a tax credit of 18 per cent on their pre-tax super contributions.

In contrast, super contributions for people earning less than $300,000 a year are  taxed at a flat rate of 15 per cent.