ROOKIE-LISTED ruckman Shane Mumford has left Geelong to begin life as a Sydney Swan.

Mumford has agreed to terms with the Swans, with the Cats handed a second-round pick (No.28 overall) in next month's NAB AFL Draft, while selection 49 is now Fremantle's.

For Geelong, Mumford's exit is a blow given the club identified the 23-year-old when he was playing country football for Bunyip and put significant development efforts into him.

However, the Cats have finally taken a hit for three years as the AFL's best team, with salary cap constraints putting the squeeze on their premiership-winning list.

"The reality is that we could not and would not compete with the dollars that Shane has been offered," Geelong's football manager, Neil Balme said.

"It's well known that we want to keep our group together and all of the players have accepted that to do that they have to forego bigger offers from other clubs to achieve that goal.

"Shane has chosen not to do that, and while we are disappointed to see him go, we will not deviate from our philosophy."

Mumford had been on a rookie contract in 2008 and 2009 but is believed to have been offered a four-year deal with the Swans worth more than $1 million.

He played 18 games this season, after three games in his debut year, and worked hard to fill the void created by injured teammate Brad Ottens.

Mumford becomes the first rookie in history to be traded during the exchange period.

In late August, the AFL announced that new regulations would permit rookies to be traded onto the senior list of another club, encouraging greater movement of players during the post-season. They were previously ineligible.

The 2009 AFL exchange period runs from October 5-9. No trades are official until paperwork has been accepted by the AFL and formally recognised after 2pm on Friday, October 9.