Moody's ratings agency on Friday warned that the economic forecasts contained in finance minister Tito Mboweni's 2020 budget had been rendered uncertain by the liabilities placed on the state by stricken parastatals and the labour movement's reaction to public wage bill reductions.

"Uncertainty regarding the success of negotiations with the country's unions and potential contingent liabilities from State-owned enterprises mean risks to budget forecasts are elevated," Moody's said.

It also noted that while Mboweni conceded that revenue would remain low, partly because of weak economic growth, he elected not to raise tax.

"As a result fiscal deficits will remain wide," it noted.

Moody's reaction to the budget Mboweni tabled on Wednesday is of keen interest to National Treasury and economists because it is the last of the major ratings agencies to maintain South Africa's sovereign rating at investment grade.

There are clear signs that trade unions will not take the minister's plans to curb the public wage bill by R160-billion over the next three years lying down.

The Congress of South African Trade Unions has termed the wage bill cut "provocative" and said it was battle-ready to oppose it.