Kirk birdied four of the first five holes on her way to a bogey-free 65 in calm conditions.
The 34-year-old said the fast start built the momentum to carry her through the round.
“Lately I’ve struggled to put four good rounds together so obviously that’s going to be the key this week,” Kirk said.
“I feel like I’m hitting it well, I’ve just got to give myself more chances and just stay patient because the birdies are out there.”
“The goal will be to keep going tomorrow and hopefully put a good weekend together. “
The news wasn’t so good for crowd favourite and five-time Australian Open champion Karrie Webb, who shot a 3-over 76.
Kirk is two shots clear of South Korea’s Chella Choi and Americans Marissa Steen and Jane Park who took advantage of calm early morning conditions to shoot 6-under rounds of 67.
“I’ve always dreamed of winning an Aussie Open so it would be nice to do it this week,” said Kirk, who is ranked 285 in the world.
“I think it would be my best win ever – as a kid you always dream about winning your national open.”Scotland’s Michele Thomson, who is playing in her first LPGA event, is a further shot back on 5-under.
“I was nervous last night trying to sleep but once I’d hit that first tee shot I felt good,” said the former policewoman.
“I just thought anything under par would be good but I never expected 5-under so I’m ecstatic and we’ll see what happens tomorrow.”
World No.1 Lydia Ko birdied her final hole to post a 2-under 71 while teenaged Canadian star Brooke Henderson shot a bogey-free 4-under 69.
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