The entire third day of the final Sydney Test was washed out Friday forcing a reappraisal of Australia's push for a series clean sweep over South Africa.

Skipper Pat Cummins has a decision to make ahead of Saturday's fourth day: whether to declare the team's first innings at 475 for four and get the Proteas in to bat or give Usman Khawaja the chance to claim his first Test double century.

Khawaja was stranded on 195 when rain ended play on Thursday, his highest Test score. Matt Renshaw, who tested positive for Covid at the start of the match, was five not out.

Another deciding factor will be the state of the Sydney Cricket Ground pitch following three days of rain interruptions.

The frequent rain and covering of the wicket has prevented the pitch from drying out and deteriorating from wear and tear for the benefit of Australia's two selected spinners, Nathan Lyon and Ashton Agar.

Australia have gone into the match with only selected two front-line pacemen - Josh Hazlewood and Cummins - making their task more even difficult if the pitch is not as conducive to spin as initially game planned.

The hosts are also pushing for a series whitewash to seal a place in the World Test Championship final in London in June.

South Africa are naturally not as concerned about the weather as they try to avoid the ignominy of a 3-0 drubbing.

"In the position we are in, the more time that is taken out of the game is probably more in our favour," spinner Keshav Maharaj said on Thursday's second day after rain brought South Africa some badly needed respite.

"But it also puts Australia in a position where they have to make a play from here on in."

The forecast is for improved conditions on Saturday with less rain expected before sunny conditions on Sunday's final day.

Australia are 2-0 up in the series after winning the opening Test by a six-wicket rout in Brisbane inside two days and then hammering the Proteas by an innings and 182 runs in Melbourne.