A queue formed to meet Sir Peter and Lady Cosgrove during the annual charity open day at Government House.

More than 3000 visitors enjoyed brilliant spring weather and free entry to the extensive grounds of the property.

However, bags were searched at the gates and media photographers were asked to demonstrate their cameras were genuine.

A circle of at least six security personnel kept watch on the crowd mingling with the vice-regal couple, while another half dozen were in the grounds close by, along with uniformed police.

Sir Peter led the International Force for East Timor in 1999 and was Chief of the Defence Force during the Iraq war.

He invited visitors inside to see the public areas of the residence, with its heritage furniture and fine art on loan, and spoke about the 400-year tree in the grounds and the lack of flies, compared with his early days at Duntroon.

"Government House Canberra might be the main residence of the Governor-General but it belongs to the Australian people and this is a unique opportunity to see one of the most historic and prominent houses in the country," he said.

Among the first to meet the couple was electrician Ben Okai and his sons, six-year-old Kwame, and Kofi, 4.

Shane Peardon said it was a fantastic experience for his family to meet the hosts.

"I've been watching General Cosgrove over the years and he's not only a great person, but a great Australian, so it's a privilege to meet him," he said.

"He was telling us about the 400-year old tree out the back which has been dropping branches over the years.

"He pointed out this tree, under which we're standing, has cables holding it together."

When the queue finally ended, Sir Peter said it was time to visit the "workers".

Nearby, William Kelly was wiping dust off the 1970 Rolls Royce Phantom, used by the Queen during her visits, by the Governor-General on formal occasions, and to drive ambassadors home after presenting their credentials.