PA Sport Magoffin had originally joined Sussex as an overseas player for the first half of the current campaign, but the county have been quick to snap up the 32-year-old on a longer deal following his decision to retire from representing his home state of Queensland.
Magoffin said: "I am very thankful to Sussex CCC, (professional cricket manager) Mark Robinson and (captain) Michael Yardy, for all of their support so far this season, and having the faith in me to continue my first-class career with Sussex CCC, beyond the 2012 summer."
He added: "I have had a wonderful time in my career with Western Australia, and most recently winning the Sheffield Shield with Queensland, and with an addition to our family due in December, this is the right time for my wife and I to relocate to the UK."
Magoffin, who took nine wickets on his Sussex debut against Lancashire last month, has previously had stints at Leicestershire, Surrey and Worcestershire in England.
