A hospital has been damaged and thousands of properties left without power by wild weather that has forced a state-wide severe warning.

The Mersey Community Hospital in northwest Tasmania was being assessed on Friday morning for the full extent of damage, with ambulances diverted to other facilities.

The hospital's clinical services were being managed but capacity was impacted, the health department said in a statement.

"Our priority is to ensure the safety of all patients and staff," the department said.

The hospital has been placed in a code yellow, for an internal emergency, with non-emergency patients being encouraged to seek alternative services.

Australian Nursing and Midwifery Federation Tasmania secretary Emily Shepherd said the union was monitoring the situation and seeking more information about the damage and staff levels.

"Our members are already stretched thin," she said.

"This situation underscores the urgent need for investment in health infrastructure, workforce planning and system-wide reform."

A severe weather warning for potentially damaging winds of more than 125km/h was in place on Friday for Tasmania, plus parts of Victoria and South Australia.

More than 20,000 homes were without power in Tasmania on Friday morning, the majority in the north and northwest.

Qantas flights scheduled to arrive in Devonport Airport on Friday have been cancelled, as was one flight from Melbourne to Launceston.

Fallen trees have blocked several roads across the state, including to the pinnacle of Hobart's kunanyi/Mt Wellington.