The trust will announce this week that it has bought one of Australia's oldest and most picturesque country mansions, Throsby Park, near Moss Vale.

The 27-room Georgian mansion was built on 1000 hectares of land granted to Dr Charles Throsby, a surgeon, settler and explorer, by governor Lachlan Macquarie in 1819.

Today the 75-hectare property - which was the subject of a painting by distinguished 19th century artist Conrad Martens - also includes a piggery, chapel, barn, mill, summer house and stables.

''The property is incredibly significant both historically and architecturally,'' said Richard Silink, the manager of the trust's Endangered Houses Fund program. ''What makes it very special is that it remained in the ownership of five generations of the Throsby family.''

The link was broken only four years ago with the death of Del Throsby. For the past 30 years the property has been maintained by the National Parks and Wildlife Service.

It will now become part of the Endangered Houses program, which seeks to find long-term sustainable uses to ensure the future of heritage-listed properties such as Throsby Park.

Far from becoming a walk-in museum, it will become a live-in house. ''The trust will undertake further work to allow the house to be lived in as a home once again,'' Mr Silink said.

The sandstone and cedar home and its colonial estate will be shown on public open days, and the proceeds from its lease will be applied to saving other heritage projects.

The move has been welcomed by John Gooch, the chairman of the Throsby Park Support Group. ''I'm absolutely delighted. The trust has an excellent record of caring for its properties.''

The group was created in 2002 amid growing concern about the state of the property.