i cleaned my whole tb today i didnt put the cables on right it ideled at just under 2k then rose to just above 2k n kept going so i turned it off fixed the cables now idels around 650ish, so like hako said check cables man
More than likely you have a faulty throttle position sensor or TPS. Black sensor located on the throttle body. Not the Idle Air Control shaped like a cylinder. The other one. If you check for errors by bridging pins 5 and 6 under the dash you'll get an error code. It helps if you have a Gregory's manual on hand.
Same thing happened to my previous car. Out of the blue one day I started it and was idling at 2K RPM. It was the TPS. I've seen them on ebay for about $30 but can get them through Holden for about $60 or $70.