The union representing Tasmania's public bus drivers is warning Friday's planned strike could be the start of a long campaign over pay and conditions.

About 300 drivers in Hobart, Launceston and Burnie are preparing to stop work for three hours on Friday morning, affecting services between 6:00am and 11:00am.

The industrial action follows a breakdown in talks between the Rail, Tram and Bus Union and the state-owned Metro Tasmania over a new three-year agreement.

Metro says its final offer of a two-per cent annual pay rise is reasonable.

Union spokesman Sam Simonettis says her members disagree, calling it an insult.

"Rude to be offered two-per cent when we deserve more than two.

"We want three, and we'll accept three."

Ms Simonettis says walking off the job is a last resort.

"We also thought about refusing to take fares, but that's part of our contract so we can't do that.

"And most people have a greencard and to stop them accessing their greencard we'd have to cover the greencard machine and then we could be charged with tampering with Metro equipment," she said.

She is warning of another strike on Tuesday, and says more action could follow:

"And that's what the members are prepared to do.

"They've said do what we need to do."

Metro Tasmania is consulting lawyers and says it regrets the inconvenience to passengers.

Most public and private schools sat they cannot afford to run extra buses to get students to school on time.