They fell from a height of about 10 metres in the freak accident during end-of-year celebrations at Hillcrest Primary School in Devonport on Thursday.

Tasmanian Police Commissioner Darren Hine confirmed two boys and two girls had died in the incident, and another five children were in hospital, four in a critical condition and one with serious injuries.

In a statement on Thursday evening police confirmed a fifth child died in hospital.

"These children were meant to be celebrating their last day of primary school, instead we're all mourning their loss," Mr Hine told reporters.

"Our hearts are breaking for the families and the loved ones, schoolmates, teachers, of these young people who were taken too soon."

Police and WorkSafe Tasmania are investigating the incident and a report will be prepared for the state coroner, who has already inspected the scene.

Commissioner Hine said the investigation would take "quite some time" and a number of people and witnesses needed to be interviewed.

"Our focus now is supporting those who are tragically affected by what's happened," he said.

Hillcrest Primary School closed for the day and told parents to urgently pick up their children not long after the incident at 10am.

Multiple ambulance crews rushed to the school and several rescue helicopters helped take the children to hospital.

Police initially reported one child had died before confirming other deaths later in the afternoon.

Counselling is being made available to families, the community and first responders.

Commander Debbie Williams said it appeared the children fell from a height of about 10 metres and described the scene as very distressing and confronting.

Bob Smith, who lives near the school, told The Mercury newspaper he saw kids on the ground.

"There was one really strong gust of wind on what (was) a beautiful calm day," he said.

"At first we thought it might have been an emergency services training exercise then the reality of what was happening kicked in."

The school was holding a 'Big Day In' celebration to mark the end of the school year.

"(The) tragedy is something that will have a deep impact on the northwest community," Tasmanian Premier Peter Gutwein said.

Prime Minister Scott Morrison described the accident as unthinkably heartbreaking.

"Young children on a fun day out, together with their families and it turns to such horrific tragedy. At this time of year, it just breaks your heart," he told reporters on the NSW Central Coast.

An online fundraising account to support the families had raised more than $24,000 by Thursday evening.

AAP