OSH would be horrified. There it is, a towering great tree over 70m high, with a double helix of 153 long metal pegs twisting upwards around its trunk, some token chicken wire stretched between them. There's no one uniformed in sight, no gates, no disclaimer forms to sign: just a weathered notice advising that "in wet weather climbing pegs may be slippery" and that sandals and thongs are not recommended footwear. Other than that, the implication is, Go for it! So I do.
The ridged spikes are well-spaced and there's a cable on the outside but, as I climb higher and higher, I become increasingly aware that it's entirely possible, if I lose my footing, to slip right through between them and plummet all the way down to the boardwalk - now rather a long way below. What seemed at first thrillingly challenging now smacks more of sheer foolish bravado.
I look up towards the crown of branches clustered at the top of this karri tree: I'm not even halfway to the metal platform 61m above the ground. I look outwards, over the wooded hills near Pemberton in this southwest corner of Western Australia. It's a pretty good view through the tree trunks, the green of the canopy shading into hazy blue towards the distant horizon, broken by patches of open farmland. In fact, the view is quite good enough, I suddenly decide, and indulge my inner wuss to join the 80 per cent of climbers who never make it to the top of the Gloucester tree.
That wasn't an option for the first 25 years of its use as a fire-spotting station: manned by forestry staff, it was somebody's job every day to clamber up this tree and others like it in the area, until planes took over the bulk of fire surveillance in the early 1970s. Of the three trees still climbable here - the Gloucester, the Bicentennial and the Diamond - only the latter is still used as a fire lookout, with the advantage over aerial patrols that there's someone keeping a continuous watch over this vast expanse of wilderness and conservation reserves, where karri, marri, jarrah and tingle trees dominate the woodlands.
"The karri tree's just like us," Wendy tells me later.
"First it grows tall, then it gets fat." She knows a thing or two about trees: it's hard not to, living in a timber town, but guiding the Pemberton Discovery Tour demands a depth of knowledge that allows her to keep up an informative commentary for several hours as we bucket along a bumpy track through the forest and then, astonishingly, over sand dunes.
The Yeagarup Dunes are a landlocked system relentlessly creeping northeast, smothering the trees in their path. It's a dislocating experience, to emerge from thick forest straight on to a mini-Sahara, the sand rippled by the wind and scribbled with animal tracks.
It's exciting, too, to lurch across and down the other side to stand by the Southern Ocean, where the waves are rolling in cold and fierce all the way from Antarctica; but even more thrilling is to be grinding anxiously back up a steep sandy track and get caught behind some youths stuck fast in a drift.
"Tch, tight tyres," mutters Wendy, getting out to advise them to let out some air; they meekly follow her instructions and, abashed, drive on. We laugh about them during our barbecue under the trees a little later.
Though the karri looks, and its name sounds, very like our kauri, it's a eucalypt and unique to this corner of Australia; so is the curiously-named red tingle tree, which grows only in an area around the town of Walpole, 130km away down on the south coast. There, in the Valley of the Giants, it's also possible to get into the upper branches of these 75m-high trees. Happily for my self-respect, this time there's no extreme climbing involved: in fact, the Tree Top Walk, 40m above the ground, is wheelchair-accessible.
Though there's also a pleasant boardwalk through a grove of these long-lived and peculiar trees, which are commonly hollowed out at the base by fire, fungus or insect damage, the main attraction is to be able to follow a cleverly-constructed metal walkway that gradually climbs up into the canopy. It's elegant and airy, built to allow some movement to convey the feeling of actually being part of the trees and, as it zigzags along for 600m, the see-through decking allows appreciation of the height above the forest floor.
It's peaceful and pretty up there, and a novelty to be eye-to-eye with the birds as I walk along, swaying; so it's hard not to tut impatiently when I get stuck behind a woman gripping both rails, white-knuckled, stiffly shuffling with baby steps, clearly petrified. I edge past, smiling but inwardly scornful: some people can be such wusses.