Hello. Given you are from Australia, it would be best to know more about your particular Celica. Could you provide your VIN (vehicle identification number) or complete frame number? I can look up the details on your exhaust system with either of these two numbers. Reason is that, here in the USA, an ST184 has the catalytic converter bolted onto the exhaust manifold, and the front exhaust pipe is bolted onto the catalytic converter, so what you have circled in red is a resonator. Your catalytic converter has studs that the flange you have circled in green goes onto (along with that gasket), and nuts that fasten the front exhaust pipe to the catalytic converter studs.
I can send you diagrams with part numbers for your specific exhaust system if you can provide your VIN or frame number.
I can send you diagrams with part numbers for your specific exhaust system if you can provide your VIN or frame number.