Jamie Briggs, a former industrial relations adviser to ousted prime minister John Howard, will represent the Liberal Party at the impending by-election for the blue-ribbon Adelaide Hills seat of Mayo.

The seat was vacated earlier this month by former foreign minister Alexander Downer, who resigned from politics to go into private business and take up a UN diplomatic posting.

He held the seat for 24 years.

Mr Briggs, 31, won a closely contested vote that went down to preferences, with former state opposition leader Iain Evans believed to have finished second in the ballot.

''This is a real honour and a humbling experience to be chosen as the next Liberal Party candidate for Mayo,'' Mr Briggs said after the vote at the city's Morphettville racecourse.

''The shoes are enormous to fill.

''We will not be taking the by-election in Mayo for granted.

''We are going to fight very hard every day up until the by-election.''

Mr Briggs, who had the support of Mr Howard and Liberal heavyweight Joe Hockey, declined to comment further on his win or the contest that lies ahead.

Mr Evans said he would continue his role in state politics and certainly stand at the next state election.

''Jamie has won a very fair fight and he has my 100 per cent support.''

Mr Downer did not attend yesterday's preselection. He is currently in New York.

On Friday the Labor Party announced it would not field a candidate in the by-election, which is expected to be held next month.

But both the Greens and Democrats will be in the running.

National Greens leader Bob Brown launched the party's campaign for the poll yesterday, with its candidate Adelaide Hills resident and school services officer Lynton Vonow declaring the environment, particularly the fate of the Murray River, to be the key issue.

''It's a real opportunity for the people of Mayo to send a message to Kevin Rudd on the Murray,'' Mr Vonow said.

''It's patently obvious that enough work has not been done yet.

''The River Murray, the [Lower] Lakes and the Coorong are dying as we watch.'' AAP