Treasurer Scott Morrison has hit back at claims the government's superannuation proposals are retrospective as the Institute of Public Affairs prepares a public campaign against the "unfair" policies.

The free market think-tank, which has close ideological ties to the Coalition, is the latest force to wade into a battle over the changes contained in the May 3 budget.

The changes target tax breaks on high-end retirement savings and include a $1.6 million limit on the size of tax-free retirement accounts, a doubling of the rate of tax on super contributions paid by people earning between $250,000 and $300,000, and restrictions on the amount of post-tax income that can be transferred.