Souths agreed to release the hooker from the last two years of his current contract at the club.
The 27-year-old was is delighted to be heading back to his homeland to play his rugby.
“I’ve been away from home for a long time now and I really miss it,” he said.
“I’ve loved playing for the Rabbitohs. They’ve done so much for me but I wanted to go back to New Zealand at some stage and I’m grateful they’ve allowed me to go.
“Going home to play for the Warriors has always been something I wanted to do. I know so many of the boys there and I also like what’s happening at the club since Jim has taken over.”
Warriors head coach Andrew McFadden is pleased to have added a player of such profile to his squad for next season.
“His arrival at the club will provide a massive lift to our side,” he said.
“With the game getting faster and the prospect of the number of interchanges being reduced, 80-minute hookers have become a real asset.
“Issac is an exceptional dummy half and he underlined that for the Kiwis in the Anzac Test against the Kangaroos. He was outstanding.
“It’s hugely exciting for us knowing Issac and Roger will on board next year.”