American rookie Ryggs Johnston claimed the biggest win of his career with a final-round 68 to win the men’s Australian Open.

South Korean former world number one Shin Ji-yai secured her 65th career victory as she won the women’s title at the event, which sees the men’s and women’s Opens played on the same course at the same time.

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

The 24-year-old world number 953 finished on 18 under at Kingston Heath in Melbourne to earn an exemption to next year’s Open at Royal Portrush in Northern Ireland.

Johnston started the final round in a share of the lead alongside Australian Lucas Herbert, two strokes clear of the rest of the field.

But Herbert struggled in the wet and breezy conditions and it was another Australian, Curtis Luck, who became Johnston’s main challenger.

Birdies on the 10th, 14th and 15th gave Johnston the edge heading home and Luck’s victory hopes disappeared after bogeys at the 17th and 18th saw him finish three shots behind the winner.

Australian pair Marc Leishman and Jasper Stubbs were third on 14 under while Herbert finished tied for fifth two further shots back

In the women’s event, former two-time British Open winner Shin held off a late fightback from South African Ashleigh Buhai, who was bidding for a third title in a row.

Shin took control to lead by eight at one point but her rival came storming back to make it within two shots.

But Shin held on to finish on 17 under with Buhai on 15 under and Shin’s compatriot, 17-year-old amateur Yang Hyo-jin, eight shots further back in third.