Sim left City for personal reasons last month, along with Latvian defender Renars Rode, who has signed with Rigas FS in his homeland.
Sydney United is a semi-professional club that competes in the second-tier of Australian football in the New South Wales section of the National Premier Leagues, essentially one step below the A-League.
"It's obviously a big club with a lot of history," Sim told FourFourTwo. "In recent years Sydney United 58 FC has been a dominant force in the NPL in both NSW and the NPL National final series. I have several ex-teammates there too."
Sim was a hit in Cape Town in the first half of the season, helping the side to the Telkom Knockout trophy.
But an injury kept him on the sidelines for a period and when he returned, he found it difficult to nail down a starting place. He asked to be released from his contract in January.
Rode will turn out for Rigas in 2017, ironically as a replacement for another former South African-based player, ex-Lamontville Golden Arrows and Ajax Cape Town centre-back Ritus Krjauklis, who has left the club to ply his trade in Malaysia.