The world's largest forest of rare Wollemi pines has been planted at Stromlo, west of lake Burley Griffin, as part of the ACT Government's $7million "tree museum".

Hundreds of the valuable trees are already dead.

The site is littered with stunted brown trees, their leaves paper-dry. Some dead trees have been removed, leaving stakes and shadecloth guarding nothing.

Thistles tower over the pines that remain on the exposed site. The arboretum has been built on hills stripped bare by the 2003 bushfires. There is no shade and no protection from the hot winds.

ANU forestry professor Peter Kanowski said about a quarter of the 850 pines had died since they were planted last year, due possibly to the water system failing, the heat, and the plants not propagating well.

"Things went off the rails ... something's gone wrong," he said.

He said the Stromlo site was "not the world's best site for growing trees".

Chief Minister Jon Stanhope has strongly backed the arboretum, saying it is a visionary project which will be a major tourist attraction. But the Wollemi deaths have been seized upon by critics, who say the project is a fanciful waste of money during a drought.

"It's a hospice for trees," past president of the Horticultural Society of Canberra Ron Gray said.

He estimated at least a third of the Wollemi pines, which are so valuable they are protected by an electric fence, had died.

The Wollemi pine, thought to have been long extinct, was discovered in 1994 in a gorge in the Blue Mountains and dubbed a "dinosaur tree". Mr Gray said it was "crazy" to plant hundreds of the pines on a hot, dry site like Stromlo.

The Government sees the arboretum as a haven for rare and threatened species, including non-natives. Mr Gray said it would be better to plant hardy species that could thrive in Canberra, like eucalypts and cypresses.

Garden writer for The Canberra Times Cedric Bryant described the arboretum as a "weed-ridden mess". "One hundred per cent they have wasted their money," he said.

Mr Bryant said the Government should have spent its $7 million on protecting existing parks from the drought. Mature trees had been ripped out because they were dying. Trees needed pruning. Yarralumla's Weston Park needed work.

Mr Bryant and Mr Gray said they did not object to trees being planted at Stromlo which was covered in thick pine forest until the bushfires but the arboretum was too elaborate and risky.