Nearly 300 patrons at a hotel in Tasmania's north-west were evacuated overnight after capsicum spray was released on the dance floor.

Police say an unknown person let off the capsicum spray at the Greens Hotel in Burnie about 2:00am (AEST), causing patrons to suffer irritation to the eyes and throat.

Two patrons were taken to hospital and ten were treated at the scene. Police are yet to find those responsible.

Sergeant Steve Jones from Burnie Police said the release of the gas was a very serious offence.

"Certainly to have first of all to have this type of spray is illegal as its classed as a prohibitive weapon under the customs act," he said.

"And it's only available to emergency personnel and correctional personnel."