The 22-year old has joined Dutch club PEC Zwolle on a two-and-a-half-year deal after emerging as one of Australia's brightest young players in recent seasons.
Sainsbury is looking forward to the next chapter of his career although the transfer is somewhat bittersweet.
"I am happy but it's going to be hard to leave the boys," Sainsbury said on Friday. "The players here are like family to me and the fans have always done the right thing by me - I've loved my time here.
"The Mariners are a real family club and while I don't know each and every supporter individually, I am sad to leave such a great bunch of people.
"If I can get myself set up there in Holland and make a good career I will be happy. The Dutch first division is big and now I have to go there and give it my all."
The municipality of Zwolle is situated in the province of Overijssel in central-eastern Holland.
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