Labor is calling on the prime minister, Scott Morrison, to urgently outline how much of the Coalition’s proposed tax cut plan will go to high-income earners, as the parties lock horns over policy costings.

Morrison was asked during Friday night’s leadership debate how much tax relief for those earning more than $180,000 would cost, but the prime minister said he could not provide a breakdown.

The shadow treasurer, Chris Bowen, said he was alarmed that Morrison was unable to answer, and has now written to Morrison asking him to request the figure from Treasury.

“If this is true, it indicates a grossly irresponsible approach to fiscal management,” Bowen said.

“As the public service is currently in caretaker mode during the election campaign, I ask that you urgently request the secretary to the Treasury to publicly release a breakdown of the total 10-year cost of your proposed tax cuts for those earning over $180,000.

“Given this election is a choice between Labor’s plan for better schools and better hospitals, or your government’s proposed tax cuts for banks, multinationals and the top end of town, it is imperative that this information is released in order for the Australian people to make an informed decision.”

Labor and the Coalition have agreed on tax cuts for middle income earners from 2019 to 2022 but Labor has not supported the Coalition’s plan to raise the 19% tax bracket threshold from $41,000 to $45,000 from 2022 then flatten tax brackets from 2024.

The demand for the government to release more details of its 10-year tax cut package comes as the Coalition questions Labor’s announcement of a new tax incentive for small business.

At Sunday’s campaign launch, the opposition leader, Bill Shorten, announced a new incentive for employers to take on younger and older workers.

Under the proposal, companies with turnover of less than $10m operating for more than two years would be eligible for a 30% tax deduction for up to five new workers’ salaries. The incentive would apply for their first year of employment, capped at $50,000 per company.

But the Coalition has said the announcement scaled back a previously announced policy outlined before the last election when the incentive was set at 40%.

The Parliamentary Budget Office costed the original plan at $257m over the forward estimates. The cost of the new policy has been revised down to $141m.