Malcolm Turnbull insists his government won't be exempting the big four banks if it succeeds in securing corporate tax cuts.

The federal coalition is edging closer to clinching crucial support from the Senate crossbench for its $65 billion package to reduce the corporate tax rate for all-sized firms from 30 per cent to 25 per cent.

Pauline Hanson's party looks likely to vote for the legislation after striking a deal for 1000 apprenticeships across the country.

If One Nation's three senators support the legislation, the government would be just two votes shy of securing support for the cuts.

Derryn Hinch is understood to favour a hike in the bank levy, initially imposed last year, if the government won't accept an exemption from the tax cut for banks.

But the prime minister says the corporate tax rate needs to be uniform.

"Because otherwise you get into problems of definition, if you say 'we are going to have a different tax rate on one sector or another', how do you define it?" Mr Turnbull told 2SM radio.

"Then you have got companies that are in two sectors, as many are."

Mr Turnbull said the bank levy was still being bedded down.

"We felt that given the support they have from government, the implicit guarantee that they have, we felt it was fair to pay an extra amount to support the government, and they did through the major bank levy, so we have already been there and raised that money from them," he said.

The second outstanding vote is that of new South Australian independent senator Tim Storer, who is working through the detail before committing to a position on the tax.

Labor leader Bill Shorten said the opposition was supporting reducing tax for small businesses with a turnover of under $2 million, as that was "fair enough".

"But a lot of big companies aren't paying corporate tax now," he told Triple M radio.

Mr Shorten talked up Labor's plan to instead extend the government's asset write-off program for business permanently.

Labor is throwing a last-minute spanner in the works by proposing a Senate inquiry which would ask big businesses to prove how they would deliver more investment, jobs and higher wages if the tax cut went ahead.

Finance Minister Mathias Cormann, who is leading negotiations, says Labor has taken a reckless and irresponsible position in opposing the cuts, and said it is a "very ignorant proposition" to argue the benefits of easing the corporate tax burden will flow offshore.

"If we make it harder for business to be successful, if we put them at a disadvantage compared to businesses in other parts of the world, there will be less investment, fewer jobs and ultimately lower wages on the back of higher unemployment," he told Sky News.

Cabinet minister Christopher Pyne said he was "hopeful" of getting the laws through the Senate next week.

"I am sure we will get there."