The federal budget has recorded a $50 billion improvement in its bottom line, but Treasurer Jim Chalmers warns it is being driven by temporary factors.

Ahead of the final budget outcome for 2021-22 being released next week, Dr Chalmers said government finances were looking better than expected.

“We’ve booked a substantial but temporary lift in taxes,” Dr Chalmers said on Tuesday.

Of the $50 billion improvement since the former government’s budget in March, $28 billion came from higher-than-expected revenues, while about $20 billion came from payments that were lower than predicted.

Dr Chalmers said the budget turnaround had been driven by higher commodities prices. But those prices were beginning to drop – with iron ore prices, for example, down about 20 per cent from their June peak and coking coal prices down 25 per cent.

COVID support payments for businesses were also lower than expected.

Dr Chalmers said there were also savings from billions of dollars in promises made under the Coalition that had not been delivered, He said they would create pressures on the budget over the forward years, particularly with delays in infrastructure, lower spending in health and social security, and pandemic-related supply chain issues.

“The large payments under spends are all about delays in COVID-related spending, including the procurement of vaccines and PPE, also delays in infrastructure spending arising from supply chain disruptions and industry constraints, as well as lower payments across health and social security as well,” he said.

The federal government will release its formal final budget outcome next Wednesday.

“But we wanted to give you a sense, because a lot of the temporary improvement that we saw in 2021-22 now becomes an issue for this government to deal with in 22-2023 and beyond – because of the overpromising and underdelivering of much of this funding that will be pushed out into subsequent years,” Dr Chalmers said.

-with AAP