A big Cameron Bancroft century and a stunning Tasmania batting collapse have put Sheffield Shield leaders Western Australia on the verge of a thumping victory at Bellerive Oval.

Tasmania capitulated to be 8-101 in a 30-over final session on Thursday, only just forcing the match into a fourth day.

A win for Western Australia would shore up their position in next month's season decider.

Western Australia looked comfortable at the crease to reach 3(dec)-372 and set Tasmania a massive 504-run victory target.

But batting seemed much more difficult when Tasmania came to the crease as the home side slipped to 5-35 in their second innings.

Opening bowlers Joel Paris and Matthew Kelly showed trademark consistency to return figures of 3-34 and 4-41 respectively.

Only opener Caleb Jewell (24), wicketkeeper Tim Paine (13), Beau Webster (21no) and No.9 Nathan Ellis (16) reached double figures for Tasmania.

WA set up their huge advantage earlier in the day with Bancroft rarely challenged by the home side's attack and continually accumulating runs.

Bancroft ended unbeaten on 176 when the declaration came, having hit 20 boundaries in a 280-ball stint at the crease.

It was Bancroft's fourth ton of the Shield season and his 19th overall.

The former Test opener went past Victoria captain Peter Handscomb - currently on tour with the Australian Test team in India - to have the most runs in this Shield season.

The contest was very much still alive at the commencement of play with WA at 1-79, leading by 223 runs.

But Bancroft and teenage No.3 Teague Wyllie batted throughout the morning session, eventually adding 147 for the second wicket.

Aaron Hardie compounded Tasmania's woes with a quickfire unbeaten 58 off 64 balls as WA moved towards a declaration.