The 25-year old Kiwi has agreed to a two and a half year deal at the Giants.
“We have been looking for quality players for a long time now and I am really pleased that Sebastine has decided that now is the time for him to come to the UK and that we are the club of his choice,” Huddersfield coach Paul Anderson said.
“He is someone we have looked at previously but he wanted to remain in the NRL – now that’s changed we will welcome him with open arms.
“We have lost a number of players from the squad for various reasons and Sebastine will bring some size, maturity and grit to our pack.
“He is a big powerful man and has plenty of his best years in front of him. He is a former teammate of Sam Rapira and Ukuma Ta’ai, they like me know that he can bring a lot to the team.
“Final clearance is expected to be finalised any day now and will hopefully be completed ahead of the Magic Weekend as our needs are obviously now and I know he just wants to get his head down and get into the rigours of Super League.
“We should also thank the St George Illawarra Dragons club for their assistance in releasing Sebastine from his contract with them.”
He can play at lock, prop and second-row.