Australia's Andrew Symonds says he is expecting variable bounce to be a big factor on day four of the second Test against South Africa.

The home side will resume on 2-110, an overall lead of 154.

Brad Hodge is unbeaten on 17 while Matt Hayden is 45 not out, having shown admirable concentration in his 163-minute stay on the two-paced wicket.

The variable bounce suggests a score of 300 would be tough for South Africa to chase on the fifth day.

Australia has an overall lead of 154 after South Africa was bowled out for 311 in reply to the home side's 355.

All-rounder Symonds, who more than doubled his career tally of wickets on Wednesday with his 3-50, is hoping to launch his batting career at Test level with a big innings at the MCG.

"It's probably more of a mind-set thing for me," said Queenslander Symonds, who is playing his sixth Test.

"I play my best cricket when I am positive but I don't want to be playing big rash shots.

"Maybe I've got to put the pressure back on the bowler and show him a bit more intent that I am going to push it along.

"It (the wicket) is going to wear quite badly, especially if it's hot tomorrow. Blokes are going to get balls that stay down or bounce or play up a bit."

South Africa's Herschelle Gibbs, who top-scored with 94 on Wednesday before becoming one of Symonds' victims, says the first session will be "vitally important".

"We need to strike early and take early wickets," Gibbs said.

"But whatever total we have to chase we're going to have to bat a day, a day-and-a-half.

"The first hour is probably going to be the most crucial in the match so far."

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