Tem received critical acclaim for his lead role as the violent Jake 'The Muss' Heke in the 1994 drama Once Were Warriors, which earned him the award for best male performance in a dramatic role at the New Zealand Film and Television Awards. He reprised the role in the sequel What Becomes of the Broken-Hearted?, leading to the Best Actor Award in the New Zealand Film Awards.
Tem had earlier become a household name in New Zealand by playing Dr Hone Ropata on television soap opera Shortland Street from 1992-95. His character entered Kiwi pop culture when he was rebuked by another character with the memorable line 'You're not in Guatemala now, Dr Ropata!'
In more recent years, Tem has spent time filming in the United States and Australia. He has played the role of bounty hunter Jango Fett in Star Wars Episode II: Attack of the Clones (2002) and re-appeared in Star Wars Episode III Revenge of the Sith. He has also had roles in other Star Wars releases. Tem's other well-known titles include Speed 2: Cruise Control and 2011's Green Lantern. He has recently finished filming New Zealand feature film Mt Zion, with Stan Walker playing Tem's son.