The Turnbull government's proposed company tax cut would drop national income for years before it boosted it and would never be self-funding, a new analysis from the Grattan Institute has found.

One of the key justifications for the proposed phase down in the company tax rate from from 30 per cent to 25 per cent over 10 years has been that it would boost national income and wages.

"Even assuming you get those things in the long run, there will be a period of time in which national income falls," said the director of the Grattan Institute's productivity growth program Jim Minifie.

"That's because you are giving a tax cut to foreigners, meaning the benefit at first goes overseas".