The tax cut for small businesses in the Abbott government's second budget will not be as effective as the government wants, economists have warned.

And the government is also likely to struggle with the 'fairness' question again this year, once all of the budget's numbers have been crunched.

The chief economist of Bank of America Merrill Lynch, Saul Eslake, says the Abbott government's decision in its second budget to cut the company tax rate by 1.5 percentage points, from 30 per cent to 28.5 per cent, for small businesses with annual turnover of $2 million or less will not be as effective as it wants.

He says despite the popular belief that small businesses are big job creators and innovators, the tax cut is unlikely to have "any material impact in stimulating stronger economic growth" because a smaller proportion of companies than expected will benefit from it.