New welfare recipients - from pensioners, to the unemployed, students and those receiving family payments - will be denied "unnecessary" carbon tax compensation under changes that will see them receiving about 1.7 per cent less than those already on welfare.

The changes will affect new applicants for welfare payments from September 20. The changes are "grandfathered" - they won't impact on those already on social security up until that date.

Treasury officials said the exact impact of the end of the carbon tax compensation was hard to precisely quantity for individuals but said they would unwind a boost to payments that amounted to 1.7 per cent. For a new single pensioner, they will forgo about $15 per fortnight compared to someone who was already on the benefit. For a new unemployed person on the standard unemployment benefit, the loss is about $9 a fortnight.

The welfare changes will also abolish the single income family supplement from July 1, 2017. The supplement allows low income, single income families to boost their tax free threshold to as high as $36,000. It was designed to replicate the tax benefit received by dual income families. Those who were eligible before July will keep the supplement.