More than one hundred plasterers have indefinitely walked off the job at Hobart's major hospital over a pay dispute but the end date for the redevelopment isn't expected to be impacted.

The 120 Chinese contract workers downed tools at the $689 million Royal Hobart Hospital project on Thursday.

Their union alleges the workers, employed by Melbourne company Accuracy Interiors, could be owed around $10,000 each and some may not have correct visas to work in Australia.

"It's exploitation of international workers not necessarily by Accurate (Interiors) but by the people who organise the labour," CFMEU secretary Kevin Harkins said, adding the workers haven't been paid in six weeks.

"We're trying very hard to sort it out as quickly as we can, but it'll take as long as it takes.

"The men need to be satisfied that they're going to be paid all their money."

Around 40 Tasmanian plasterers working at the site have been paid correctly, Mr Harkins said.

John Holland-Fairbrother, the construction company in charge of building a new impatient area at the hospital, believes its mid-2019 completion date won't be affected, Project Director Ben Moloney said.

The employment of other subcontractors was being reviewed to ensure national and state obligations are being met, he added.

"We are surprised of these issues being raised," Mr Maloney told reporters.

Health Minister Michael Ferguson described the pay issues as "very concerning" and is pushing for the project to be completed on time.

Accuracy Interiors has been contacted for comment.