Australian Car Repair Issues? Ask an Auto Mechanic for Answers ASAP
Sounds like a head gasket issue to me.
I would conduct a test for CO (carbon monoxide) in the coolant.
If this passes, and depends on the mechanic, I would then monitor the cooling system pressure with a gauge while driving.
I would replace the thermostat as well, because they DO go faulty once they have got too hot.
Some things for your mechanic to think about.
Do you need any further help?
I hope my advice has been of assistance to you.