Investec economist Annabel Bishop predicted Nene would cut National Treasury's February forecast of year-on-year GDP growth of 2.7 percent to 1.5 percent.

She added this had been expected, but meant the minister had to guard against government's direct debt rising as Moody's rating agency had warned South Africa risked a further downgrade if it was allowed to escalate in an unsustainable manner.

"Should the Medium Term Budget Policy Statement deliver higher net debt ratio projections, or fiscal slippage with the fiscal deficit projected to reach 3.0 percent of GDP after 2016/17, then SA could receive a further credit rating downgrade, which would then cause the rand to weaken closer to our down case."

But Bishop said communication from Treasury ahead of Wednesday's medium-term budget had signalled a commitment to fiscal efforts to avoid this scenario.

Bishop said as the 2014/15 financial year reached its halfway mark, government spending was slightly higher than budgeted compared to 2013/14, and revenue collection somewhat lower.

It was therefore expected that Nene could give some information on potential future tax changes.