A fire truck to replace the one damaged will be delivered to the brigade on Thursday or Friday from Sydney.

Jerrabomberra Creek RFS brigade captain Brad Griffin said those involved had received support from NSW and ACT RFS.

"They are recovering now, and in due course they will make a full recovery," Mr Griffin said.

Four firefighters were involved but were out of the truck when the tree fell. Three were physically injured. They were assessed on site by an ACT Ambulance service who was stationed nearby. The four firefighters were taken to the Canberra Hospital for assessment before being released.

The incident happened on Orroral Road, near Orroral campground, at the south east of the fireground about 10.45pm during patrols of the Angus fire trail.

Mr Griffin said the crew had been on a routine tasking to assist the ACT RFS, and "these things go wrong".

He said the truck was out of action after a medium to large sized tree fell on to the back of it.

That meant the brigade was two trucks down, after one rolled on the Kings Highway two weeks ago on the way back from a fire.

"It was a single vehicle motor accident, the truck swerved to avoid a deer," Mr Griffin said. No one was injured.

The first damaged truck had been temporarily replaced, and one from Sydney was due within 48 hours to replace the one damaged by the tree.

Mr Griffin said his brigade has 43 members, and over the past 60 days they had done about 7000 volunteer hours. They have been fighting fires since August when they were tasked up north.

That equates to every volunteer member giving up about 20 hours a week to help fight fires.

Mr Griffin said the current conditions were reminiscent of 2003.

"It's deja vu," he said.

"It's a concern, but we've been doing this long enough to know we're as prepared as what we can be."

"We've done a huge amount of preparation, both at the local level, the district level and also at the state level... We're doing grader clearing along the railway line as we speak," he said.

Mr Griffin urged the community to be prepared.

"Don't panic, stay calm, be alert, not alarmed," he advised.

NSW RFS spokeswoman Angela Burford said the four crew members had been onboard a category seven tanker, a smaller version of a large fire tanker.

ACT Policing interviewed firefighters at the scene.

ACT Emergency Services Agency Commissioner Georgeina Whelan said crews had been briefed not to go more than 10 metres into bush areas beyond fire trails and roads due to the risk of falling trees.

"Firefighting is a dangerous business and the face the crews were adhering to safety briefings minimised risk occurring," Ms Whelan said.

Fire crews have been working to contain the out-of-control bushfire burning in the Namadgi National Park, before conditions deteriorate late this week.

The Orroral Valley fire is five kilometres from Tharwa and eight kilometres from Banks in the southern part of Tuggeranong. The blaze has burnt 10,829 hectares as of 4pm Wednesday and continues to grow in size. It was downgraded to advice level at 4pm.

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The horror bushfire season has been a dangerous one for firefighters, several of whom have lost their lives in recent months.

Americans Captain Ian H McBeth, 44, First Officer Paul Clyde Hudson and Flight Engineer Rick A DeMorgan Jr, 43, died a week ago when the C-130 Hercules firefighting aircraft they were flying crashed in the Snowy Monaro region.

Earlier this month, father-of-two and 40-year veteran firefighter Bill Slade, 60, was struck by a tree while working at the edge of a fire in Victoria's alpine region on Saturday, January 11.

Volunteer firefighter and father to be Samuel McPaul died while battling the Green Valley fire on December 30. He was involved in a freakish weather event with extreme strong winds lifting the 10 tonne fire truck off the ground, landing on its roof.

Geoffrey Keaton, 32, and Andrew O'Dwyer, 36, died on December 19 when a tree fell into the path of their tanker, causing it to roll, as they were travelling in a convoy near Buxton, southwest of Sydney.

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