Munster not sure if he'll replace Slater in Melbourne No.1 jersey

He's played fullback for the Storm in a grand final but Cameron Munster says Billy Slater's shock retirement doesn't automatically mean he will wear Melbourne's No.1 jersey next year.

Munster filled in at the back for the best part of two seasons while Slater recovered from injuries but there's now a host of contenders to make that position their own.

Over the past 12 months he has represented Queensland and Australia at five-eighth and while he enjoyed his previous stints at fullback, he isn't sure exactly which position he'll fill in 2019.

"When Bill came back, I was a bit worried to play his position and didn't know how well I'd go at it," the 23-year-old said.

"Now I've played Origin and got to a World Cup because of that position. It's going to be tough for myself. I'm not really worried about next year, but it's something I'm going to have to worry and think about. We've got some good players in Jahrome, Scott Drinkwater and Ryan Papenhuyzen coming through.

"We've got Val Holmes and Kalyn Ponga playing good footy for that Queensland fullback role. If I want to change, I've got to do some stuff and probably lose a couple of kilos too."

Munster was appreciative of how Slater, while injured, taught him about fullback play despite being a potential rival for the role.

"There was a lot of pressure going into Bill's shoes when he did his shoulder," he said.

"I was nervous going into that role, and he worked on a couple of things with me in defence, and a little bit in attack. I was surprised how well he wanted me to succeed in that position. He could have been kicking stones back then, but he wanted the club to succeed and really focused on us winning."

Slater's retirement announcement on Wednesday came as a surprise.

"It was a bit shocking, to be honest," Munster said.

"He's playing some great footy for us. Me personally, I thought he could go another year, but Billy knows his body better than anyone, and everyone's time comes to hang up the boots, and he feels next year is the time."

And although there's motivation to send out Slater as a premiership winner, the team's focus is still to improve their recent form.

"It's obviously a goal for us to go back-to-back, but we've got to worry about our own backyard at the moment. We haven't been playing well - we lost to the Rabbitohs last week - and we weren't very good in some key parts of our game."