'Lethal Weapon' Johnston and Shin Ji-yai win Australian Open titles

Fearless American rookie Ryggs Johnston fired a 68 to win the Australian Open on Saturday, with South Korean former world No 1 Shin Ji-yai crowned women's champion for her 65th career title.

Johnston sank an eagle and five birdies to three bogeys, finishing at 18-under for the tournament to claim his maiden professional win at the DP World Tour event in Melbourne.

It earned him an exemption to next year's British Open.

He outlasted Australian Curtis Luck by three shots and Marc Leishman and Jasper Stubbs by four.

Named after Mel Gibson's character Martin Riggs in the Lethal Weapon movies, Johnston turned professional this year and earned his playing card through the DP World Tour qualifying school.

"I turned up pretty tired already from the travel and Q-school and I didn't get a practice round with the weather," he said.

"So I didn't really have any expectations which probably helped me in the end.

"It hasn't quite sunk in yet, but an amazing feeling."

In contrast to Johnston's inexperience, former two-time British Open winner Shin has been on tour for nearly 20 years, but she had a nervy finish.

The 36-year-old was seven clear with eight holes to play before South African two-time defending champion Ashleigh Buhai came storming back to get within two shots.

Buhai eventually finished second with Shin's compatriot, 17-year-old amateur Yang Hyo-jin, eight shots further back in third.

The mixed Australian Open involved men and women teeing off in alternating groups on the same courses.

Johnston and LIV Golf's Lucas Herbert were the men's overnight leaders by two strokes.

But Australian Herbert struggled in the wet and breezy conditions at Kingston Heath in Melbourne's famous sandbelt region and fell off the pace.

Luck was the day's big mover, surging into a one-stroke lead with his putter running hot.

But birdies on the 10th, 14th and 15th gave Johnston the edge heading home and when Luck made bogeys at the 17th and 18th his challenge was over.

Shin started two clear of Australian world No 6 Hannah Green and four ahead of Buhai, setting up a showdown between three major winners.

An anticipated grandstand finale looked to have fizzled out early on with Shin taking control.

She mixed three birdies and an eagle with a double bogey in her front nine to build a six-stroke lead from Buhai, which soon stretched to seven.

Green imploded, dropping six shots in her opening nine holes, to slump out of contention, but Buhai surged back to give Shin a fright.

"I have only one Australian Open trophy and I'm so excited to get hold of the next one, especially at Kingston Heath," said a relieved Shin, who also won at Royal Canberra in 2013.

"I had a battle with Ashleigh and I just tried to focus on my game and it was not easy, but now I can breathe."