Nick Xenophon has offered Malcolm Turnbull billions of dollars in budget savings, as a ratings agency warned the government to step up and deliver.

The prime minister is on track to be able to govern with a slim majority in the lower house, but will need to rely on Senate crossbenchers to deliver some budget measures and election promises.

Senator Xenophon, who addressed a business conference in Brisbane on Friday, looks set to have a team of three senators and a lower house MP.

He told reporters the government should keep in place the levy on Australia's highest income earners to balance the budget, instead of axing it as the coalition pledged in the 2016 budget.

Labor also supports retention of the levy, which it says will return $4.2 billion to the budget over four years or $19.2 billion over the decade.

"The so-called temporary deficit levy needs to be extended until we get the deficit under control," Senator Xenophon said.

He said politicians and others earning more than $180,000 a year should not find it too onerous.

The South Australian senator also called for the Medicare levy to be lifted "a smidgin".

Further benefits would come from giving the auditor-general greater power to scrutinise government spending, including payments made to the states.

The comments came as Standard & Poor's called on the government to achieve its forecasts.

"There's been a number of years of fiscal slippage and it's really time for the government to step up and deliver on what it's telling us," S&P associate director Anthony Walker said.

The ratings agency this week cut Australia's ratings watch to negative from stable, warning the country will lose its triple-A rating in two years if the government doesn't act.

About $12 billion in previous budget savings have not yet been legislated.

Prime Minister Malcolm Turnbull wants the first session of the new parliament to legislate middle-income earner tax cuts and a reduction in the company tax rate for small businesses under $10 million turnover.

He'll also seek support to wind back superannuation concessions and crack down on welfare fraud in a bid to secure a budget surplus by mid-2021.