Tasmanian senator Jacqui Lambie says it's worrying that the government wants to legislate tax cuts that won't kick in for years.
Crucial Senate crossbencher Jacqui Lambie has suggested she might push for changes to the Morrison government's flagship income tax cuts.
And another crossbencher says he's not bothered if the cuts don't pass parliament until later in July.
The coalition's $158 billion tax package passed the lower house on Tuesday and is heading for the Senate, where the government needs support from four of six crossbenchers to get it across the line.
Senator Lambie is yet to declare if she will support the full three-stage package, and is worried about the final stage, which doesn't kick in until mid-2024.
"They are five or six years away and that's a bit worrying because we don't know where the economy's going," Senator Lambie told reporters in Canberra on Wednesday.
"They're big calls. I would like to put a safety procedure in that stage three."
The senator is not worried that amending the bill could hold up tax relief for Australians.
"I'm watching it play out, you know what politics is like. It changes more than you change your undies."
The government is in the market for crossbench support because Labor won't support the tax package in full.
It's pulling out all stops to get the laws passed this week, pledging to keep politicians in Canberra for as long as it takes.
The pair of Centre Alliance senators are almost on board, but still have a few final details about gas prices they want to sort out with the government to make sure any tax cuts aren't gobbled up by higher power bills.
Senator Rex Patrick said he hadn't met with the government since publicly voicing those final concerns on Tuesday morning.
"It's with the government at this point in time," he told AAP.
"If this gets held over to the next sitting week, in our view that's ok, people will still get their tax cheques.
"Having said that, I think we'll end up getting there."
Parliament is next due back in Canberra on July 22.
Former Liberal Cory Bernardi also backs the tax relief package, leaving the government just one vote short.
The Labor opposition is hoping it can convince crossbenchers to support amendments so the second stage can happen sooner and the third stage be voted on later.
Labor argues the third stage - set for 2024/25 - is too far off for the parliament to decide on now.
"This is a government which has got the economy wrong at every turn," shadow treasurer Jim Chalmers told reporters.
"(It) now wants to pretend that they know what the economy and budget will look like in five years time."
Treasurer Josh Frydenberg's is piling pressure on Labor.
His latest tactic is to highlight how much the nation's lowest-earning electorates will miss out on if the plan doesn't pass this week - more than $15,000 over a decade in Bruce in Melbourne's southeast and Blaxland in western Sydney.
The first stage of the plan will deliver up to $1080 to low and middle-income earners when they lodge their tax returns in coming months.
The second stage tops up a low-income tax offset, meaning people earning up to $45,000 - instead of $41,000 - will have a 19 per cent tax rate.
The final stage flattens the tax rate from 32.5 per cent to 30 per cent for people earning between $45,000 and $200,000 from mid-2024.