Forestry Tasmania managing director Bob Gordon today said the swamp gum (eucalyptus regnans), dubbed Centurion, stands between 100 and 101 metres tall.

"It is the only known standing hardwood tree in the world to be over 100 metres tall," he said.

Mr Gordon said it was the tallest known tree in Australia.

It is the world's tallest eucalyptus tree, the tallest hardwood tree, and the tallest flowering plant, he said.

Centurion is 405cm in diameter and its height was measured using laser survey equipment.

It was discovered in a state forest near the Tahune Airwalk tourist attraction, 80km south-west of Hobart.

A second giant swamp gum named Triarius, standing 86.5 metres tall with a 390cm diameter, was found alongside Centurion.

Forestry Tasmania officer David Mannes said Centurion may have been taller in the past.

"It appears to have broken off at the top, then re-sprouted a new healthy crown," he said.

The two trees will be safeguarded under the state's Giant Tree Policy, which provides immediate protection for trees taller than 85 metres.

Previously, the tallest known hardwood tree was Icarus Dream, a swamp gum measured at 97 metres tall, in the state's Styx Valley about 100km north-west of Hobart.

Forestry Tasmania says records from Victoria, mostly from the 19th century, describe a number of trees taller than 100 metres, but many of those measurements are disputed.

None of those Victorian giants now exists, it says.

Californian redwood trees grow taller than Centurion, but they are softwood trees, and botanists do not classify them as flowering plants.