Wealthy Australians earning above $300,000 per year could be hit with a new minimum income tax rate under a Shorten Labor government after the ALP agreed to consider implementing the so-called "Buffett rule" for the first time.

The change, promoted by key left-wing frontbencher and former leadership aspirant, Anthony Albanese and adopted unopposed, is designed to ensure that high wealth individuals who use creative accounting to minimise taxable incomes are subject to a flat rate - perhaps as high as 35 per cent of their "total incomes" - which would not be negotiable.

The Buffett rule gained that name when the philanthropically inclined multi-billionaire US investor Warren Buffett discovered he was paying less in income tax than the middle-income staff he employed.