Eight independent MPs led by Allegra Spender have called on the federal government to index income tax thresholds to inflation, a major challenge ahead of an election in which both sides are using bracket creep to fund their promises.

The letter signed by Spender, Helen Haines, Sophie Scamps and the five other teal independent MPs elevates the issue of tax reform ahead of an election due by May 17, which could return a hung parliament with those MPs in the balance of power.