The highest income earners in Australia would be forced to pay a mandated minimum rate of tax under a policy proposal being pushed inside the Labor Party to counter the populist appeal of One Nation and potentially add $2.5 billion a year to the budget bottom line.

The push by the Left could mean Opposition Leader Bill Shorten goes to the next election promising a so-called "Buffett rule" requiring people earning more than $300,000 a year, for example, to pay no less than 35 per cent of their taxable income.

That would mean the richest 30,000 households would be forced to contribute an average $79,000 a year more in tax than they currently do, according to research by the National Centre for Social and Economic Modelling.

The idea has been dismissed as "class warfare" by advocates for corporate Australia but senior figures in the union movement believe Labor must respond to the rise of right-wing populism and its focus on inequality with policy positions that until recently were considered "outside the political orthodoxy".