S&P on Monday said the COVID-19 pandemic would strip billions in forecast revenue from the NSW budget while also forcing the state government to undertake large infrastructure and fiscal stimulus programs.

The NSW government's general revenue forecast over the four years to July 2024 has been written down by $14.8 billion, including some $8.7 billion in foregone GST grants due to reduced consumption.

Some $4.3 billion was saved through curbing public-sector wage growth.

S&P said NSW's outlook remained stable but revised the state's long-term credit rating to AA+, while its short-term credit rating was maintained at A-1+.

The agency forecast that NSW debt serviced by taxation would rise to 149 per cent of operating revenues by July 2023, up 71 per cent over four years.

Even the proposed privatisation of Sydney's Westconnex motorway and potential sale of NSW Lotteries could not maintain NSW's AAA credit rating.

"Our ratings on NSW are supported by its wealthy and diversified economy, which is recuperating from a recession in the first half of 2020; excellent financial management; and exceptional liquidity," S&P said in its report.

"NSW was quick to control the spread of coronavirus, averting what could have been a deeper recession. It is well-placed to return to operating surplus and stabilise the balance sheet impact in the medium term."

It comes after fellow credit agency Moody's last week maintained its AAA rating for NSW, despite an erosion in the state's financial strength.

NSW Treasurer Dominic Perrottet said in a statement on Monday he hoped the recent state budget would eventually help return NSW to a AAA rating.

"Our economic response has been about creating as many jobs as possible, supporting businesses through this time and maintaining record level of investment in infrastructure," Mr Perrottet said.

But NSW shadow treasurer Walt Secord said S&P's rating change was "devastating" and augured poorly for the state's economy.

"NSW unemployment is still (at) 6.5 per cent and at least 281,700 people are unemployed ... NSW used to be the engine room of the Australian economy and now we lag behind the other states," Mr Secord said in a statement.

Total spending in NSW jumped 6.8 per cent in the September quarter of 2020, having fallen by 8.8 per cent in the pandemic-affected June quarter.

But last month's budget revealed a $16 billion state deficit, while the NSW economy is expected to shrink by 0.75 per centin real terms in 2020/21.

S&P also on Monday downgraded pandemic-hit Victoria's triple-A credit rating to AA.

Australian Associated Press