Australia's highest income earners will be slugged a top marginal tax rate of almost 50 per cent under an alternative proposal put forward by Labor to fully fund the National Disability Insurance Scheme, and which the government will be under pressure to adopt.

The proposal - crucially supported in-principle by the Greens and the Nick Xenophon Team - involves the 0.5 percentage increase to the Medicare levy as outlined in Tuesday's federal budget but only for people earning more than $87,000 a year.