Marsh produced another heroic knock on Wednesday, but NSW are still in the box seat in the JLT Sheffield Shield clash at Optus Stadium.

In reply to NSW's 261, WA were in trouble at 4-68 following a top order collapse.

Marsh responded with a gutsy 81 off 193 balls to guide WA to 5-188, before a collapse of 5-10 saw the hosts dismissed for 198.

NSW moved to 1-32 in their second innings by stumps on day two - an overall lead of 95 - with Nick Larkin on 15 and Kurtis Patterson on nine.

Marsh has been in impressive form since returning from his poor Test series against Pakistan, producing scores of 80, 98, 163no and 81 for WA, as well as his 106 for Australia in an ODI fixture against South Africa.

He said a relaxed attitude had helped him regain his mojo.

"I've just got to keep having fun out there," Marsh said.

"I'm just really relaxed and not putting so much pressure on myself, and I'm going out there and playing with a smile on my face.

"It's all I've done over the last three or four weeks.

"I haven't worried about what's been written about me, I haven't been worried about anything really. I've just gone out there and enjoyed it."

Marsh's knock against NSW was as patient as it gets, with the left-hander careful not to take many risks on the new Optus Stadium wicket.

The veteran survived strong lbw appeals when he was on 28, 43, and 48 before bringing up his half-century off 152 balls.

Ashton Turner (38) combined with Marsh for an important 80-run stand that helped steady the ship, but their good work was undone by the late collapse.

Paceman Trent Copeland finished with 4-52, while spinner Nathan Lyon claimed 3-34 from 21 overs.

Day one belonged to WA paceman Jhye Richardson, who snared career-best figures of 8-47 in a mesmerising display of pace and swing.