Ashton Agar celebrated his 21st birthday with a man-of-the match performance to help Western Australia rally for a 28-run victory over Tasmania at North Sydney Oval on Monday.

Off-spinner Agar scored 64 off 60 deliveries, sharing a seventh-wicket partnership of 79 with Nathan Rimmington (55) after WA had been struggling at 6 for 141.

Agar, who had made a remarkable 98 batting at number 11 in his first Test appearance for Australia last year, also took 1 for 40 off eight overs on Monday, securing the important wicket of middle-order batsman Jordan Silk.

"I just tried to be nice and positive and just play good cricket shots," Agar said.

"It was a hard situation as I had to work myself in, but I knew if a few of us could get going we could post a good score.

"I felt really good (bowling). I have done a lot of work during the winter dedicated to my bowling and it feels like that is starting to pay off now.

"I feel like I know my game now. Last season was a massive learning season for me, and I have taken a lot out of that."

At 3 for 176 Tasmania appeared to be cruising to victory chasing 268 but the loss of top-scorer Ed Cowan (76) ended a 99-run partnership with Silk and the Tigers went on to lose their last seven wickets for just 64 runs.

Joel Paris and AJ Tye each took three wickets for Western Australia.

The West Australians had struggled to keep the run rate ticking over for much of their innings under cloudy skies, but some late hitting resulted in 93 runs from the final 10 overs to boost their tally to 8 for 268.

WA made the worst-possible start with Craig Simmons out to the first ball of the innings, while Marcus Harris was also out for a golden duck an over later.

Key batsman Michael Klinger survived an early run out appeal by the narrowest of margins before eventually compiling 42.

Evan Gulbis was the pick of the Tasmanian bowlers with figures of 3-45, while Andrew Fekete also impressed with 2 for 43.

It is the third win on the trot for the second-placed WA who now have one hand on a finals' berth, while Tasmania have just one win from three outings.

AAP