Withers, 32, joined the Robins at the start of 2012, and has gone on to make 41 appearances.
He said: “Myself and my family will be heading back to Australia at the end of the season.
“I’m undecided as to whether or not I’ll be carrying on playing footy back home or maybe starting a new chapter in my life. I pride myself on my fitness levels and will to win and still feel as though I have plenty to offer as a player so we’ll see what happens.
“I’ve made some great friends through rugby league and would like to thank them all for their support and friendship. I’d also like to thank my family as well because without their support, everything I’ve achieved wouldn’t have been possible.”
The former Canberra, Wests and St George man first came to Super League with Crusaders in 2009.