The South African prevailed after a dramatic collapse from Sydney prodigy Grace Kim to dash local hopes for another year on Sunday.
Buhai closed with a one-over-par 73 at Victorian Golf Club to post a nerve-wracking one-stroke victory over Korean Jayai Shin (75).
With a winning 12-under 72-hole total of 277, Buhai completed a rare British Open-Australian Open double, having also triumphed at Muirfield to clinch her maiden major championship in July.
Half an hour earlier, 21-year-old Kim had stood on the 18th tee sharing the lead.
But the new LPGA Tour graduate had to settle for outright fourth at nine under after making a mess of the par-5 18th to chalk up a soul-destroying double bogey.
Celebrated compatriot Hannah Green (74) birdied the last to pip Kim for third at 10 under.
But Green's hopes had already evaporated on the front nine.
After starting the day two shots off the lead, she recorded two doubles and a bogey to spiral from 12 to eight under and out of contention, six strokes adrift of Buhai heading to the back nine.
Fellow heavyweight Minjee Lee (71) finished fifth at eight under but also must be wondering what might have been.
With an ugly double-bogey seven at the eighth, she spoiled a golden start of three birdies in six holes that had rocketed the world No.4 into contention.
The US Open champion was never a factor after falling seven shots behind Buhai at the turn.
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