Top income earners aged over 65 could end up being taxed at a higher rate under a new proposal to help cut back the future cost of New Zealand Superannuation.

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The option came out of Auckland University's Retirement Policy and Research Centre, in research for the Commission for Financial Capability, as it looked to review retirement income policies.

The suggestion would see NZ Super payments remain a universal grant, but when extra income was earned, it would be taxed under a progressive system so that high-income earners effectively paid back the grant.

The interim retirement commissioner Peter Cordtz said the idea was worthy of consideration.

"Tax has always been about the redistribution of wealth, so it's worth looking at ideas where income could be redistributed more equitably, particularly between generations."

In one scenario, a tax rate of 39 percent would be imposed which mean superannuitants earning over $123,000 annually would cover their NZ Super grant with their tax payments.

There would also be a lower rate of 17.5 percent which would protect the majority of over 65s who earned only a modest extra income.

"Reductions in payments to high income superannuitants may release more taxes to fund services that benefit the population as a whole," Mr Cordtz said.

The researchers found the proposed tax regime, had it in been in place in 2017-2018 would likely have saved around 16 percent of the cost of NZ Super.