Exactly one year ago on Wednesday he boarded a bus for Sydney on the first leg of a journey halfway around the world on his trusty single cylinder Honda CT 110 or ''postie bike''.

Mr Carmody, who returned to the ACT with his bike three spark plugs, 23 oil changes and 23,000 kilometres later on New Year's Day, has been chosen as the ambassador for the National Trust-sponsored rally which is being officially launched by centenary creative director, Robyn Archer, at 1pm on Wednesday.

''I've never been an ambassador before,'' he said. ''I feel humbled. I will be taking part in the rally of course. Just look at it; what a ride. We start at HMAS Creswell at Jervis Bay and travel to Canberra through Mt Sassafras, Tarago and Queanbeyan.''

Canberra's last motoring event on this scale, 2001's Federation Rally, attracted more than 1300 vehicles from across the country. Organisers are hopeful the centenary rally will attract 2000 or more.

Mr Carmody, who stepped down from the ABC last year, spent seven months crossing America from Los Angeles to New York before flying to London where he then rode to Istanbul.

''It was a great adventure,'' he said. ''You only live once and you don't want to have any regrets (about things you have not done).''

Last year's epic motorcycle odyssey was not his first big adventure. ''In 2005 my wife, Sandra, and I took the year off and rode pushbikes around Australia. I chose the postie bike this time around because it is a similar experience to the push bike but can carry a fair bit of weight and covers ground more quickly.

That said, neither he nor the bike broke any speed records. ''The top speed is about 80km/h and I usually cruised along at between 60 and 70km/h,'' he said. ''This meant I had to avoid freeways and live life on the B roads. It was a great way to meet people; they were drawn to the bike especially in America.

''I was at a place called Salmon in Idaho - if you ever go to Idaho you have to go to Salmon - and I was riding back to my camp,'' he said. ''Suddenly there was a police car behind me with the lights flashing and the siren going. A very large policeman got out and said to me 'in this State we've got a minimum speed limit and a maximum speed limit and you're not doing either of them'.''

Mr Carmody, who timed his northern hemisphere travels to coincide with the spring, summer and autumn, kept costs to a minimum by camping out as often as possible. He said people were almost universally friendly, more so in the US than Europe, and that the bike was a great ice breaker.

''America is full of big Harleys but the riders aren't snobbish about it at all. Almost everybody I saw on a bike there gave me a wave.''

The National Trust Centenary of Canberra Rally is being organised by the Council of ACT Motoring Clubs.

It is open to all types of vehicles, regardless of age, but participants must be members of a car club or the National Trust and the vehicle must be registered and roadworthy.