Australia's tax agency says it can retrospectively deliver tax cuts if the coalition's proposal doesn't pass through parliament before the end of the financial year.

But it remains to be seen if the tax cuts will pass parliament, with key crossbench senators still to pledge their support for the plan.

Government senator Zed Seselja is urging Labor to support the changes in order to give them a seamless course through parliament, avoiding the need for the crossbench votes.

Shadow treasurer Chris Bowen says Prime Minister Scott Morrison is already breaking election promises, after pledging Australians tax relief this financial year.

"If the Australian people have to wait another year for the tax cuts, I think it's an indictment on his government and the character of the prime minister," he told reporters in Sydney on Tuesday.

Greens leader Richard Di Natale says the minor party won't support the income tax cuts.

The party held onto all of its six Senate seats up for re-election, taking the Green's total to nine.

"We had millions of Australians voting for parties like the Greens in the Senate to hold this government to account, and we'll do that," he told ABC radio.

"We're not going to support tax cuts to people on half a million dollars … if any support is going to be given it needs to be targeted at people on low incomes."

The Australian Tax Office says it can retrospectively amend tax assessments to provide cuts if the laws pass after June.

The agency could also make administrative changes to provide tax cuts.

"If the Labor party agrees to support the coalition tax cuts as announced, then we would be able to update the tax withholding schedules, to allow the tax cuts to be reflected in people's take home pay," the ATO says on its website.

Treasurer Josh Frydenberg will meet with Treasury officials and the Australian Securities and Investments Commission in Canberra on Tuesday, before heading to Sydney to catch up with Mr Morrison.

He's expected to meet with Reserve Bank of Australia Governor Philip Lowe and the Australian Prudential Regulation Authority on Wednesday.

The meetings come as Dr Lowe prepares to deliver a major speech on the outlook for the Australian economy and interest rates in Brisbane on Tuesday.