Poor Sydney weather cruels Labuschagne

A frustrated Marnus Labuschagne has been left to rue Sydney's poor weather after being denied a fourth century of the summer on a rain-interrupted opening day of the third Test against South Africa.

Labuschagne was caught behind for 79 off a vicious, rising ball from Anrich Nortje, before the players immediately left the field for a second time due to bad light with Australia 2-147 and stumps was called.

The right-hander's dismissal came at the end of a brief four-over stint after an earlier two-and-a-half hour delay, mostly due to poor light with some rain also falling in patches.

"I was definitely very frustrated," Labuschagne said.

"It always makes you very angry as a batter when you get out and then everyone walks with you off the field.

"It really makes you think about that specific ball was probably under the light meter and you're getting out."

Asked if he felt it was too dark when Nortje had bowled the ball, Labuschagne said he had not considered it until his dismissal.

"I was just really focused on the ball ... It certainly was dark," he said.

"The umpires need to make sure if they do feel like it is dark, it doesn't matter if the over is still going or that something happens before we go off.

"But perhaps they thought until I was dismissed (it was OK) and then the light dropped suddenly and it became too dark."

Having already wrapped up the series, a win for Australia on a Sydney wicket already offering spin will guarantee qualification for June's World Test Championship final.

But 47 overs in, it already appears likely the weather - which has also impacted six of the last seven SCG Tests - could present the biggest hurdle for the hosts.

More rain is forecast for parts of Thursday, Friday and Saturday.

Labuschagne had looked in impeccable touch before his exit, pulling and cutting his way to 79 as he took to Kagiso Rabada and Keshav Maharaj after lunch.

His only other scare came on 70 when he edged a ball off Marco Jansen and was given out on soft signal, before third-umpire Richard Kettleborough deemed the ball had bounced before reaching Simon Harmer.

South Africa were left furious and remonstrated with umpire Chris Gaffaney and Labuschagne, with the latter adamant the ball had bounced.

"All of us thought it was out. Simon was convinced it went straight in," Nortje said.

The chance came late in Labuschagne's 135-run stand with Usman Khawaja (54no), who himself passed 4000 Test runs on Wednesday.

Now averaging more than 100 at the SCG, Khawaja pulled the ball nicely and drove neatly through the covers.

David Warner was the only Australian to fall in the opening session when he edged the in-form Nortje (2-26) to first slip on 10 as he chased a wide delivery.

Australia's other Wednesday drama came in the form of a COVID-19 case, with Matt Renshaw testing positive shortly after being named for his first Test in four years.

He plans to play on and bat at No.6 while using a separate changeroom, with Peter Handscomb or Marcus Harris the likely COVID substitutes if Renshaw becomes too unwell.