The 30,000 cut in the immigration intake will cost the federal budget $500 million a year, a Treasury briefing obtained by Fairfax Media shows.

There were 162,000 permanent visas approved in the 12 months ended June 30, a cut from 190,000 the previous year, which Home Affairs Minister Peter Dutton described as evidence "the policy is working as it should". The Turnbull government has framed immigration as a key issue for Saturday's byelections, as it road-tested key policies before the next federal election.

Home Affairs Minister Peter Dutton. Credit:Justin McManus

Between April 2017 and April 2018 Treasury produced two reports to Cabinet on the financial impact of a cut to immigration. The period was marked by disagreement between Prime Minister Malcolm Turnbull and Mr Dutton over whether ministers had discussed slashing Australia’s immigration intake from 190,000 to 170,000 a year.

Those documents will not be released under a freedom of information request, but Fairfax has obtained a Treasury briefing showing how it calculates the contribution of individual migrants to government revenue.