Winterstein, 27, signed for the Bulls on a one-year deal from Widnes back in October, but suffered a pectoral injury in Samoa‘s opening World Cup game against New Zealand, ruling him out of the start of the new Super League season.
The former Wakefield and Crusaders man returned to his native Australia ahead of the Christmas period, got married and has since opted to stay Down Under, where it is understood he is likely to join a rugby union side.
Bulls coach Francis Cummins said: “I am disappointed to lose anyone, but when I met Frank he did talk about an opportunity for him to play rugby union in Australia.
“He was keen to play out one year with us here, in Super League, until he got back home.
“Him and his wife have just got married and I guess, for a player who has in the past witnessed financial difficulties at other clubs, the uncertainty wasn’t great for him.
“If an opportunity has arose elsewhere for him then we cannot really stand in his way, given the fact we are attempting to make cuts to our playing budget.
“It is an unfortunate situation, but I believe we have adequate cover at second-row. He certainly added to our strength in depth, but there are four or five guys ready to fill that gap.”