Before spotter planes, drones and satellites, Western Australia relied on humans posted in treetop lookouts to detect bushfires.

In bushland surrounding Perth and in Dwellingup and Harvey, there were numerous hilltops and rocky outcrops suitable for these lookouts, according to bushfire management consultant Roger Underwood.

But the ancient karri forests around Manjimup and Pemberton, where the trees were 50 metres tall and the land was fairly flat, presented a particular problem — finding a lookout spot that was higher than the trees.

"The first step in bushfire control is fire detection; actually having some means of detecting fires when they start and locating and reporting them," Mr Underwood told 720 ABC Perth.

The Diamond Tree lookout in 1997. ( Flickr: John Evans )

"The original foresters, when they were trying to get a system of fire detection going down there, they were stymied.

"There was no place where they could build a lookout.

"And that's when they hit on the idea to use the trees; instead of being the problem, they would become the solution."

Two karri tree lookouts still climbable

Between 1937 and 1952 nine treetop lookouts were built in the karri forest.

Only two of the trees are still able to be climbed — the Diamond Tree and the Gloucester Tree, which are both over 50 metres tall.

The Diamond Tree ladder is made of 130 steel pegs. When the lookout was built they were made from wood. ( Flickr: Amanda Slater )

Their ages are not known but both trees are estimated to be at least 250 years old.

Bushfire heroes celebrated

On Saturday a group of enthusiasts will gather at the base of the Diamond Tree to mark 75 years since it became a lookout.

They will also celebrate a number of "great heroes ... that were associated with the amazing development of fire tree lookouts," Mr Underwood said.

"The first of these was Don Stewart who was a local forester down there.

"He was the person who conceived the audacious idea of putting a ladder way up a tree and putting a cabin on the top of it and linking it with telephones and having someone looking up there for fires."

The second is his colleague Jack Watson, who climbed 40 trees to find the highest and most suitable for use.

The view from the Diamond Tree lookout allowed fire watchers to see smoke over the surrounding area. ( Flickr: Jimmy Chen )

"It's quite an amazing feat because with karri trees, they are not just tall, but they have few branches," Mr Underwood said.

"It's not like climbing from one branch to another, you have to work your way up.

"Karri bark is notorious because when it is wet it is very greasy and slippery ... these guys used to have very long fingernails."

The third hero is Dick Sprogue who was famous for trimming the karri branches.

"He was the guy that used to clamber around in the tops of the trees and chop the branches off," Mr Underwood said.

"If you can imagine being up there, 200 feet from the ground, on a swaying tree, wielding an axe, chopping the branches off a tree — these guys were tough and they were fearless."

Treetop climbers warned

The eight karri tree lookouts were used by spotters every summer until 1973.

"Of the original nine trees, one called Big Tree came to a sorry end, burnt down in a bushfire, although it didn't happen when someone was up in it," Mr Underwood said.

Another in Pemberton lost its lookout during Cyclone Alby in 1979, while several others rotted and died.

Both the Gloucester and Diamond treetop lookouts have been restored and are climbed by thousands of visitors every year.

The base of the Diamond Tree bears a sign warning would-be climbers to consider carefully before attempting the climb. ( Flickr: doc44 )

The Department of Parks and Wildlife warns climbers to think carefully before taking on the climb, which is almost vertical towards the top of the tree.

While no-one has ever died undertaking the climb, two people have had heart attacks after climbing the trees, the Pemberton Visitors Centre website warns.