Michaela Thom, 2, from Farrer with grandparents Leonie and Doug Thom from Port Maquarie.

Michaela Thom, 2, from Farrer with grandparents Leonie and Doug Thom from Port Maquarie. Photo: Jeffrey Chan

The National Arboretum will change as it matures but already its lookout offers a new panorama of Canberra.

''It is a different perspective,'' Port Macquarie resident Doug Thom said.

''We haven't been before, it's something you can't see anywhere else … it's just spectacular.''

From the lookout on the top of Dairy Farmers Hill, there are views east over Lake Burley Griffin and of the city's leafy inner-south, including Parliament House and the old Yarralumla brickworks. Looking west are the Brindabellas while the Black Mountains are to the north.

The arboretum open day on Sunday, the last before the official opening on February 1, brought Canberrans and visitors out in force to explore the 250 hectare site.

During the mid-afternoon the lookout was fanned by strong warm easterly winds. ''It looks like the hot air is coming from that building,'' Mr Thom said, pointing to Parliament House.

A patchwork of saplings is expected to lure 350,000 visitors to Canberra in the first year, according to Territory and Municipal Services.

Plans for an international arboretum go back to 1915 but the catalyst was the devastating bushfires in 2001 and 2003, after which the ACT government decided to replant the area and create the National Arboretum.

In addition to attracting visitors and providing a new area for Canberrans to explore, the arboretum will be used by the Australian National University for research into climate change.