Out in the first group on Saturday, Girrbach cashed in on the relatively easy conditions before heavy rain set in to post a seven-under-par 65 at waterlogged Kingston Heath.
The 31-year-old world No.445 nabbed the third-round clubhouse lead at 10 under for the championship.
Halfway pacesetter Lucas Herbert teed off just before 11.30am (AEDT) with a four-shot lead at 14 under.
A defiant Cameron Smith insisted a maiden Open crown remained in touching distance despite suffering a sour end to his second round on Friday.
Smith entered the third round eight strokes behind his LIV Golf teammate Herbert.
Following a blistering start on Friday, the former world No.2 slumped down the leaderboard to a tie for 18th after imploding on the back nine.
But Smith said all was not lost, particularly given the rain forecast to pound Melbourne's sandbelt.
The 2022 British Open champion believed he was still playing well enough to challenge.
"I'm playing really good golf. It was just kind of a shocking nine holes and a couple of mental errors where I felt like I was running around there a little bit," he said.
"There's still a lot of golf to be played and that's the way we've got to look at it.
"And be aggressive out there but also be quite patient because there is a lot of birdies out there so I don't need to do anything different."
Smith began well but came unstuck with a horror back nine on Friday. (Con Chronis/AAP PHOTOS)
While Smith struggled on Friday, golf's new golden boy Elvis Smylie sparkled while surging up the leaderboard.
The winner of last week's Australian PGA Championship carded a sublime 64 to rocket into a share for third at nine under.
Smylie had been three over through seven holes on Thursday, but has since amassed 13 birdies to roar back into contention to complete a rare summer double of twin victories at Australia's two flagship tournaments.
Hyojin Yang is a surprise halfway leader in the women's division. (Con Chronis/AAP PHOTOS)
The stage is also set for a thrilling finish to the women's Open.
Teenage amateur Hyojin Yang was the shock halfway leader at nine under.
But the 17-year-old has fellow South Korean, former world No.1 and 2013 champion Jiyai Shin and home hopes Justice Bosio and Hannah Green breathing down her neck.
Shin and Bosio are just one stroke behind, with Green two back and looking ominously ready to pounce and crown her stellar 2024 with a fourth title and cherished maiden Open crown.
"I've put myself in a good position for the weekend," said the world No.6.
Fellow major winner Minjee Lee was the big mover on Saturday morning, picking up four shots to surge to one under for the tournament through 15 holes.
Lee will be hoping the field backs up in the more difficult afternoon conditions so she can challenge for a first Open trophy on Sunday.
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