The Turnbull government is continuing to insist that its tax package must be considered as a job lot, and it will get through the Senate intact, despite the obvious parliamentary resistance to the third tranche of the reforms, which flattens the income tax scales, benefiting high-income earners.



The treasurer, Scott Morrison, said on Sunday the government was often underestimated when it came to dealing constructively with the Senate, and he said the plan needed to be legislated as a whole because it dealt with both short and long-term problems.

Morrison is also continuing to resist calls to release detailed, year-by-year costings of the tax measures beyond the forward estimates, contending the figures would be “unreliable”.

Labor has made the case the government should not ask the parliament to sign up to a seven-year plan without a clear understanding of the costs to the budget once the benefits begin to flow to high-income earners, when the government scraps the 37% tax bracket.

The Grattan Institute has produced analysis demonstrating the top 20% of income earners will benefit most from the tax plan Morrison outlined in the budget last Tuesday. Its analysis found that once the third phase of the plan was in place, $15bn of the $25bn annual cost results from collecting less tax from the top 20% of income earners.

Morrison told the ABC on Sunday that if Labor wanted year by year breakdowns of the tax package, then it needed to produce year-by-year breakdowns of its promise to end cash refunds for excess imputation credits for individuals and superannuation funds.

“So when people want to apply the same standards to the government that they do to the Labor party, fair enough, but on this score we have supplied the clear costing over 10 years and Labor is looking for [an] excuse not to provide tax relief for Australians because they don’t believe in it,” the treasurer said.

The government last week unveiled a new low and middle-income offset of $200 for people earning under $37,000, rising to $530 for more than four million people earning between $48,000 and $90,000.

It also proposes to increase the 37% tax threshold from $87,000 to $90,000 before axing that bracket altogether in 2024-25 – a development that would see people earning between $41,000 and $200,000 paying the same marginal tax rate.

The package costs $13bn over the forward estimates and $140bn over the medium term.

The government has the requisite support for the first stage of its plan but Labor and a number of Senate crossbenchers, including the One Nation bloc, have been cool on the last phase on the basis it makes the tax system less progressive.

Bill Shorten used his budget reply speech last Thursday to promise he would almost double the rebate for low and middle-income earners and he signalled strongly Labor would not support phase three of the government’s plan. Labor will also impose a 49% rate for people earning over $180,000 if it wins the next federal election.

The competing tax plans will face their first on-ground political test during a super Saturday of byelections over the coming month or so triggered by the last vestige of the dual citizenship crisis.

An opinion poll released on Saturday suggests the Coalition has a good chance of taking the Queensland seat of Longman from Labor because of the high One Nation vote, and it could also win back the South Australian seat of Mayo from the Centre Alliance MP, Rebekha Sharkie.

The Liberals will not field a candidate in the two Western Australian seats in play, Fremantle and Perth, to save resources for the federal election, which can be called anytime after August.

The byelection season is looming as a major test for both major party leaders. A government has not taken a seat from the opposition of the day at a byelection for a century.