The 27-year-old was on the radar of both Motherwell and former club St Johnstone, but has instead opted for a new start in Australia.
City reached the semis of the A-League finals last season behind the goals of ex-Ibrox youngster Ross McCormack.
O’Halloran is currently in talks to see an early release from his contract at Rangers. He played only once for the club last season, as a 89th minute substitute in a 2-1 win over Hearts, after falling out of favour with Pedro Caixinha and then Graeme Murty.
The speedster initially joined in January 2016 for a £500,000 fee after impressing for St Johnstone in a 3-1 victory over Mark Warburton’s men in a League Cup clash at Ibrox.