Centre Alliance Senator Rex Patrick wants to be reassured the federal government's promised tax cuts won't short-change essential services down the track.
That comes as Labor remains concerned that legislating the later stages of the tax plan, as they'll kick in four years down the track when the state of the economy remains to be seen.
Senator Patrick's centrist minor party is likely to hold two seats in the upper house after the May 18 election, which means it could play a key role in getting the three-stage tax cuts across the line if Labor doesn't back them.
Senator Patrick and his upper house colleague Stirling Griff are comfortable with the first two stages of the plan, which will put more in the pockets of low and middle income earners.
But they remain unsure about the later stage of the government's $158 billion package, which will flatten the tax rates schedule by mid-2024.
The coalition has ruled out breaking up the legislation, insisting the long-term structural changes are pivotal.
"We have concerns that if the economy softens, then we'll end up in a situation where we have a lack of revenue to support essential services such as education, health, aged care and pensioner support," Senator Patrick told Sky News on Sunday.
"That's the due diligence that we're going through at this point in time."
Senator Patrick says he's asked the government for protections that would ensure the long-term tax relief wouldn't be chewed up by rising power prices in the future, if gas costs grow.
"We might have a tax cut that simply goes to energy companies."
The opposition is also weighing up whether to support the later stages of the government's tax package, saying it will come to a position before legislation comes to parliament next month.
Labor leader Anthony Albanese is uncomfortable that the structural changes to the system won't come into effect for several years.
"Given the softness in the economy, it is a triumph of hope over economic reality for the government to say it knows what the economy will be like in 2024/25," he told reporters in Perth on Sunday.
He's also urging the opposition to bring forward some of it's planned spending on infrastructure to help keep the economy moving.
"What we need to do is actually have some stimulus right now."
Asked during the week whether the coalition could consider that prospect, Treasurer Josh Frydenberg said the government is always seeking to bring projects online, but that there are always boxes to tick.
"We will bring the projects online that we can," he said.
