It wasn't as emphatic as hoped, but Deyen Lawson has held on to claim the WA Open and a maiden PGA Tour of Australasia title by two shots from Michael Sim.

The Victorian started the final round eight shots clear at 21 under and after 11 holes was 24 under and eyeing the tournament's greatest-ever victory.

Then the nerves set in.

But not even two bogeys and a double bogey on the run home - he dropped five shots in six holes - and a Sim birdie on the last was enough to derail an otherwise dominant performance.

A one-over round saw Lawson (62, 63, 64, 71) finish 20 under, while Sim's equal-best round of the day - six-under 64 - earned him outright second at 18-under at The Western Australian Golf Club.

It meant Lawson fell well short of Kel Nagle's tournament record 11-shot win in 1951, while Ryan Fox's 23-under winning total in 2014 remains the high-water mark.

"It's not easy. It never is," Lawson, a four-time runner-up on the PGA Tour of Australasia said.

"Sometimes if you're that far in front ... you try not to have thoughts creep in but I just kept pushing and pushing.

"Through nine I might have got nine shots ahead and then thought, 'now just hang on'. Which was not the right thing to do.

"In future I know that if I've ever got a good lead I need to just keep pushing until I finish. Really keep firing."

Queenslander Chris Wood (66) was third at 16 under and Sydney amateur Jeffrey Guan (68) finished fourth, his best finish in a professional event.