It was a chilly late November afternoon in Hobart in 1999 when the Wilderness Society's Geoff Law received a call that would turn his day on its head.

On the phone was Bob Brown, the Tasmanian doctor, environmentalist and former Greens leader who was one of the founders of the Wilderness Society.

"Bob said, 'I've come up with an idea,'" says Law, who was the Wilderness Society's Tasmanian campaign manager at the time.

Brown's idea was for the Wilderness Society to decorate an 80-metre-tall eucalyptus in the Styx Valley with fairy lights and other decorations to make it the "world's tallest Christmas tree".

He hoped the decorations, the fanfare and media coverage would bring national attention to the logging of the old-growth trees in the Styx Valley, a forest home to some of the world's tallest trees.

But the logistics of getting decorations, thousands of fairy lights, power lines, generators and people out to this remote area several hours north-west of Hobart — let alone getting them up an enormous gum tree — was not lost on Law.

"I was just in stunned silence," he says. "Almost against my better judgement, [I said] 'Ah yes, we can do that.'

"In those few moments, I committed the Wilderness Society to taking on this ambitious, audacious, half-crazed project."

The start of the campaign

The Christmas campaign relied on Law's long experience in running campaigns for wilderness areas that were special to him.

Law was a shy, self-confessed "nerd" when he first joined the Wilderness Society as a volunteer for its iconic founding campaign to save the Franklin River in Tasmania in 1976.

He had loved the natural world since he was a kid, and quickly found himself getting his hands dirty to prevent a hydroelectric dam being built on the river.

"I was impressed that this group was … projecting confidence and optimism and seemed to be fairly mainstream. I didn't want to be involved with a group dominated by hippy types," Law says.

After the success of the Franklin campaign in 1983, Law ascended to the top of the ranks as the Wilderness Society's Tasmanian campaign coordinator in the next decade.

And a year before Brown pitched his idea for the Christmas tree, Law and Brown visited the Styx Valley together.

"We got out of the car and walked into the intact forest. And then the world changes," Law remembers.

Once in "an almost silent area of intense greenery and shade," they were confronted with the huge eucalypt trees, some as high as 100 metres.

"The thing that strikes you first of all is not the height of them, but the width of them.

These were ancient trees, some of them 300 or 400 years old. And they were under threat from logging.

Knowing that these old-growth trees were lucrative for logging companies and were threatened provided plenty of motivation to act when Brown came up with the Christmas tree idea.

Decorating the tree

"The first thing we had to do was find someone to take on the climbing side of things and also to find the tree," Law says.

The tree they eventually chose was 80 metres tall — higher than a 26-storey building. But attempting to scale a tree of that height came with significant challenges.

Ben Ray was chosen to be the climber, someone Law describes as athletic and "a natural leader".

To get up the Christmas tree, he first climbed up an adjacent tree that had branches at lower levels and was therefore easier to climb.

Ray managed to scale to a height where he could put ropes and slings across to the first branch of the giant eucalyptus Christmas tree, and then eventually make his way up through the sets of branches.

But trying to string fairy lights along the steep branches 26 storeys up a tree at that time of year was dangerous in Tasmania.

"December in Tasmania is a time when we can get very inclement weather," Law explains.

"Periods of wind and rain and hail and snow on the surrounding peaks. [There were] times when the tree and the climbers in it would be buffeted by 80 kilometre per hour wind gusts, and the tree as a whole would sway and twist, and the branches would be thrashing around."

It wasn't just the physical challenges they were up against.

Law's team was trying to decorate one of the world's tallest trees without drawing attention from Forestry Tasmania, the state's public forestry company, which was logging in the area.

And Law was trying to recruit people for the clandestine operation to decorate the tree at a time of year when most were busy doing Christmas shopping and getting ready for the holidays.

His team had a long list of last-minute Christmas shopping, too.

"In order to make it look like a real Christmas tree, we had to have fairy lights [and] a star on the top of the tree, [and] there had to be presents at the base of the tree," Law says.

"We strung up more than 3,000 fairy lights, and each of them had to be connected via power boards and extension cords to a generator. There were beach balls which were made to look like dangling Christmas baubles. There were huge boxes wrapped in Christmas paper to look like Christmas presents at the base of the tree."

The finishing touch: "Santa Claus abseiling his way down the tree saying 'Ho ho ho, Merry Christmas!'"

Multiple media outlets and TV cameras are present for the unveiling of the tree on December 20, a few days ahead of Christmas.

On the top of the tree was its crowning glory: a giant star with four-metre-long arms made of fluoro tubes and long pieces of aluminium.

And, like a family Christmas lunch, expectations were high.

"We all nervously waited for the moment when the switch would be flicked, and [to see if] it would either light up or it wouldn't," Law says.

A simple flick of a switch belied the group effort put into transforming a Tasmanian eucalypt into the world's tallest Christmas tree for the day.

"The lights illuminated and there, on the top, was this wonderful big star against the evening sky. And the whole thing looked fantastic," Law says.

"My busker friend was at the base of the tree with his piano accordion singing 'O Christmas tree, O Christmas tree'."

"People were giving whoops of joy. There was a lot of exhilaration and, on my part, a huge amount of relief."

Saving the Styx

Even though they pulled off a minor Christmas miracle, they found that ensuring the Styx Valley was safe didn't happen overnight.

After the Christmas tree stunt, there were more protests and campaigns, as well as a Guinness World Record for the world's longest tree sit, which lasted 449 days.

It was over a decade before the valley was protected. Law was in the audience when UNESCO announced in 2013 that much of the Styx, along with the upper Florentine and Weld Valleys, were to be added to Tasmania's World Heritage Area.

There is still logging in Tasmania: giant trees in the Florentine and Little Denison are still being logged.

But what happened in the Styx Valley can be a shining light for other parts of Australia, Law says.

"We can all draw strength from some of the things that have been achieved in the Tasmanian wilderness, which seemed like hopeless causes at the time, but have resulted in the protection of magnificent stretches of natural countryside, mountains, caves, rainforests, winding rivers, Aboriginal heritage and rock art, all of which make up one of the great wild areas on the face of the planet."

A few years ago, Law went back to the Styx to find out if his Christmas tree was still standing.

"I dropped into one of the reserves and was very pleased to see it was now managed not by Forestry but by the Parks and Wildlife Service," he says.

"It was by no means crowded, but it was nice to see that there were people enjoying these ancient giant trees that could so easily have all been completely flattened, burnt, destroyed, and very few people would have been the wiser about it."