Australia will witness the ''managed extinction'' of one of its rarest mammals, Leadbeater's possum, unless the federal government intervenes to save its old growth mountain ash habitat, a leading scientist says.

Australian National University ecologist Professor David Lindenmayer has written to federal Environment Minister Tony Burke this week, requesting the possum's conservation status be upgraded urgently from endangered to critically endangered under federal biodiversity protection laws.

''Unless we move quickly, we'll see this animal go extinct within 25 years. If governments do nothing, then Leadbeater's possum is stuffed,'' he said. The possum, named after Victoria Museum taxidermist John Leadbeater, was thought to be extinct until the mid-1960s, when a colony was discovered living in forests near Marysville - one of the areas hardest hit by the Black Saturday bushfires in 2009.

Professor Lindenmayer, who has studied the possum and its habitat for 30 years, estimates there could be as few as 500 left.

''We are quick to accuse other countries, like Indonesia, of clearing wildlife habitat, but we need to look at our own backyard first. What we are doing to our native forests and wildlife is just as bad,'' he said.