Prime minister Malcolm Turnbull may yet deliver on his promise to cut tax for all companies, with two senators dealing themselves back in to talks.
The federal government is two votes short in the Senate of passing the legislation to take the tax rate from 30 per cent to 25 per cent by 2026/27.
It was thought the government's only hope lay in two independents - Victoria's Derryn Hinch and South Australia's Tim Storer - giving the bill the green light.
But Rex Patrick and Stirling Griff of the Centre Alliance, formally known as the Nick Xenophon Team, are still doing "due diligence" on the tax cut, despite initially opposing the plan.
Senator Patrick met with senior Treasury and Reserve Bank officials, tax commissioner Chris Jordan, tax reform guru Ken Henry and others in his quest to inform himself of the pros and cons of the cut.
"The starting point is 'no', but in fairness to the government whenever we are dealing with any piece of legislation we will go off and do due diligence," Senator Patrick told AAP on Wednesday.
"It's no different to other legislation, I go out and talk to people on both sides of the question. I venture down every nook and cranny. Then I prepare a brief and go back to my party and the party makes the decision."
The NXT delivered crucial votes for the initial tax cuts for businesses with turnovers under $50 million.
Senator Patrick said one of his key concerns is the impact on gross national income, which he said would "dictate spending cuts in the future".
He is also worried about rising energy costs, which some companies say are a greater priority than tax cuts.
Senators Hinch and Storer are also seeking advice before making a final call.
Senator Hinch doesn't want the big banks rewarded for their bad behaviour as outlined in the royal commission, and Senator Storer has called for broad-ranging tax reform to be considered.
One Nation supports the tax cuts, but leader Pauline Hanson also wants the banks' share to go towards compensation for misconduct.
The government delayed putting its legislation to a vote in the upper house just before Easter, admitting it didn't have the numbers.
The ACTU, Business Council, BHP, EnergyAustralia, MYOB and Fortescue Metals will give evidence to a Senate committee in Melbourne on Thursday examining the tax cut's economic benefits.
