OLYMPIC swimming legend Murray Rose will launch a 12-month countdown to the Sydney 2009 World Masters Games tomorrow.

About 30,000 competitors arrive next October for the biggest sporting event in the city since the 2000 Olympics.

"This is as big as it gets," Rose said after a swim at Bondi.

"To put it into perspective, the Sydney Olympics attracted 10,000 competitors."

Organisers expect athletes from more than 100 countries to compete across 28 sports at 70 venues, including Olympic sites. Most competitors are aged between 35 and 55, but for sports such as swimming the minimum age is 25.

"One of the great things about these games is that it's not about elite athletes. Everybody gets to represent their country at an international sports event, just as long as they qualify on an age basis," Rose said.

The State Government estimates the games will generate up to $68million for the state economy.