Australian Cars did not have the Volvo receiver. They have an Aftermarket unit. It fails.
Some have a complete after market alarm integrated.
The simplest way is replace the receiver with a Volvo US or UK unit.
You need to replace the receiver unit under the glove box.
You can then easily get fobs and program them as per the Volvo method.
If you want an immobilizer then that is a separate relay. The receiver will trigger it. Be aware there are different ones. I can't remember which one I installed I will post back later.
I have not gone down the road of the higher spec alarm/relay or repairing the after market units.
The manufacturer of the Australian unit is no longer in businesses so I doubt you will get support from the dealer.
So the bits you need are
Receiver part no. 3545654 FCC cwt5654r
Fob part no. 3545653 fcc. CWT5653T
Are a matching set.
It also appears
Receiver part no. 9148645 fcc no.CWTTFWB1G
Fob part no. 9148646 fcc no. CWTTFWB1G
https://ozvolvo.org/discussion/1655/keyless-door-opener#latest