Reigning Dally M medallist Cooper Cronk will leave the Melbourne Storm to move to Sydney at the end of the 2017 NRL season.

The premiership-winning half-back re-signed with Melbourne in August last year, on a deal that was supposed to take him through 2018 with the club.

That two-year extension, however, gave Cronk the freedom to make his own choice for the 2018 season and he opted to move for personal reasons.

Cronk's fiancee, Fox Sports journalist Tara Rushton, lives in Sydney and the playmaker said he was spending every spare second flying between the cities, and it was getting hard to manage.

He said there was more to life than football and he had realised how much he wants to start a family.

"It's time I put someone else and something else first," he said.

Splitting time between Melbourne and fiancee Tara Rushton in Sydney was getting hard to manage for Cronk. ( AAP: Paul Miller )

"I'm jealous of the guys who have their families here ... and it's time for me to try and prioritise my time and my future."

From back-up to the best of the best

Cronk started his career with the Storm in 2004, joining Cameron Smith and Billy Slater, with whom he also played lower-grade football at Brisbane Norths.

With Matt Orford the team's star half-back, Cronk plied his trade as a utility off the bench before Orford left in 2006 and the Brisbane native was handed the starting seven spot.

Cronk made it his own, having developed one of the game's most pin-point kicking games and has earned 19 State Of Origin caps and played for the Kangaroos 32 times.

Cronk (right) has played football with Cameron Smith (centre) for more than 15 years. ( AAP: Paul Miller )

Despite being perhaps the lowest-profile member of the 'Big Three', Cronk won the 2013 Dally M Medal and shared the award with Jason Taumalolo in 2016.

He was named best on ground as the Storm won the 2012 premiership.

"To Craig [Bellamy], Cameron and Billy — people talk about the 'Big Three' — they're the real big three. I'm a distant relative," he said.

The announcement came after round five and Cronk said he would rather make it public so the club can avoid months of speculation and start making moves to replace him.

"Whoever does come in, I promise I'll give them the keys to the city and tell them everything I know," he said.

Cronk showed his trademark humility at his press conference, but coach Bellamy made it clear he was not just another player.

"This is going to be a very, very hard guy to replace," Bellamy said.