Tributes have flowed in for volunteer Mid-Canterbury firefighter Barry Keen, who was killed in the line of duty today.

Mr Keen, 52, died early today when a burning tree branch fell on him as his crew turned up at a stubble burn-off at Willowby, near Ashburton.

Transalpine fire region officer John Barnes said the crew turned up to find a single tree burning in a paddock.

The branch appeared to have fallen and hit Mr Keen on the head as he walked around the fire truck.

Ashburton Mayor Bede O'Malley told Radio Live he had since heard it appeared the noise of a pump may have masked the noise of the branch falling.

It was also dark as it was 5am, and there was a significant amount of wind around at the time.

Mr O'Malley said Mr Keen, who left a wife and son, had been a volunteer firefighter for 35 years, and besides his experience, he was also a very "careful and meticulous individual".

He said the accident had hit Mr Keen's family and colleagues hard.

"They are in shock at the moment and coming to grips with the event..."

Mr Keen was also involved with the Ashburton IHC branch, having been president for many years until recently, and had been instrumental in getting the organisation to the strong position it was in today, Mr O'Malley said.

Ashburton IHC president Donald Thompson described Mr Keen as a loyal volunteer and supporter of the organisation, who played a valuable role in fundraising and branch activities.

Those associated with the branch were devastated, he said.

Mr Thompson said for 20 years Mr Keen was the area convenor for the IHC calf scheme, and in September 2006 he was presented with an outstanding voluntary contribution award along with his wife, Rose.

Other colleagues said Mr Keen was a "good bloke" and that those at the scene of this morning's incident were receiving support.

The incident is being investigated by the Department of Labour, Fire Service and police.