Bill Shorten has declared the Labor Party's position on ending refundable tax credits is "not for turning", saying there was no logic to them.

Facing growing pressure from self-funded retirees who face to lose up to $5 billion a year in refunds, the Opposition leader said he would stand by the policy.

The government is ramping up attacks on Labor's plan which would end a tax practice introduced by John Howard and Peter Costello at the turn of the century.

People who have no taxable income receive a refund from the ATO for the franking credits on their shares in publicly listed companies.