The dealer's mechanic should be able to connect the computer to a read out and determine from the error codes stored in the ECU,
what the problem is. If he cannot, I would find another Subaru
mechanic. Is the car running OK? Sometimes these lights come on simply because the driver has not put the fuel cap back on tight; i.e., evap emissions. I don't know what the requirements are for emissions in Australia but this is a frequent complaint in the states. The solution is to tighten the cap after every fill and the ECU will reset itself after several starts and the check engine light should go off.