In doing so, the 2022 British Open champion became the first back-to-back winner since Taiwan's Tseng Ya-ni in 2010-11.
The 35-year-old carded a three-over-par 75 to end at nine-under for the tournament, a stroke ahead of Lee whose 69 ensured a tense finale.
Buhai began a blustery day at the Australian Golf Club in Sydney with a three-stroke cushion over South Korean veteran Shin Ji-yai and seven clear of world number five Lee.
"To be honest, I wasn't feeling the pressure, I knew Minjee and Ji-yai would come at me today," she said.
"But the conditions were so tough, I just said hit it into your spaces, play par golf and make them catch you."
Shin birdied the first and the third to move within a shot of Buhai, before a bogey at the sixth again left her two adrift.
Two more dropped shots from Shin, coupled with the South African parring every hole on the front nine, sent Buhai to the turn with a four-stroke lead.
But two-time major winner and playing partner Lee was in fine touch and reduced the deficit to three shots with six holes left.
A disastrous double bogey at the 13th appeared to end Lee's charge, but the pressure got to Buhai who bogeyed the 13th, 14th and 17th to set up a nervous final hole with her lead down to one stroke.
Lee came up short with a long birdie putt that could have forced a playoff as Buhai sunk a three-footer for par and back-to-back Australian Open wins.
Shin collapsed to a 77, ending tied for third with fellow South Korean Jenny Shin, five behind Buhai.
The mixed Australian Open involved men and women teeing off in alternating groups on the same courses.
The Jack Nicklaus-designed Australian Golf Club was the main venue across all four days, while the nearby Lakes Golf Club co-hosted play during the first two rounds
