The prospect of a tax rise on the superannuation earnings of wealthy Australians is emerging as another difficult issue for Julia Gillard, with former chief whip Joel Fitzgibbon joining other Labor MPs speaking out about their concerns.

With the May budget approaching, Labor needs to find billions of dollars to pay for programs such as the school funding reforms and the national disability insurance scheme.

The Prime Minister on Wednesday again refused to rule out taxing more from the superannuation earnings of wealthy Australians; but Mr Fitzgibbon worries that Labor might botch the definition of what constitutes a ''wealthy Australian''.

''In Sydney's west you can be on a quarter of a million dollars family income a year and you're still struggling,'' Mr Fitzgibbon told Fairfax Media.