Australia Olympic team head Nick Green would not promise to equal or even surpass the country's number three finish in the Beijing edition of the summer games in 2008 but he is upbeat in crashing the Top Five circle.

Mr Green made his bold projections as the whole world started on Wednesday the 100-day countdown for the London Olympics, which is set to host competing athletes from around the world on July 27.

It will be the 30th edition of the modern Summer Games, when 204 countries will contest the thousands of medals at stake in 26 different sports and 39 various, according to Wikipedia.

Australia, Mr Green said, will be represented by 400 athletes, which he added is the country's smallest Olympics contingent over the past two decades.

The country's medal campaign will be bannered by the swimming and cycling team, though Green noted that both Aussie swimmers and cyclists would face tough challenges from their rivals.

The local Olympic chief pointed to the United States and China as the likely closest foes of Aussie tankers while bikers from the United Kingdom are expected to give local riders definite good runs.

The swimming team will be plunged in the fight without the traditional flairs that were previously displayed by Australian pool legends Ian Thorpe and Michael Klim.

Instead, the 44-strong swimming team will be carried by James Magnussen, whose heroics in the recent 100-metre freestyle world championships would be the basis of the targeted 12 gold medal haul for the team, head coach Leigh Nugent told the Agence France Presse (AFP) on Wednesday.

Apart from the U.S. swimmers, "there are other players who are making it tougher for us to stay in that number two position. The big movers are China and France," Mr Nugent added.

The odds were also stacked against the Aussie riders, Mr Green conceded, citing the results of the recently concluded world championship held in Melbourne two weeks ago.

Clearly in that event, the UK team got the better of their Aussie counterparts as they clinched top wins in five contests while the latter only managed three wins, Mr Green said.

"Although the cycling may level itself out, we anticipate we can make some gains in other sports at the same time," he added.

Australia's overall Olympic quest was further hurt by the elimination of the country's football teams, both the men and the women.

"I think top five is the respectful and rightful position for Australia to aspire to ... (But) we are in for a tough tussle to claim a revered top-five spot with hosts and long-time rivals Britain, the strong European contingent headed by Germany and France, and the ever-present Japanese," the Aussie Olympic chief told AFP.

"Our athletes have to continue pushing in these final 100 days to give themselves the best chances of Olympic glory and ensure that we will hear Advance Australia Fair playing throughout the London venues," Mr Green stressed.