Flannery who turns 32 later in June, is out of contract at the end of the season and intends to return to his native country, Australia.
He joined Saints back in 2007 as a replacement for Jason Hooper and has since made 98 appearances for the club.
He said: “I think this will be my last year.”
“Hopefully, I’ll have a big year this year and finish it off in the right fashion.”
“I think I’ll hang the boots up,” he said. “I’ll go back and get some of that sunshine, to the house on the beach and do a bit of surfing. I’m looking forward to it.”