The specialised buses are too big to fit in normal lanes.
Seriously? We are using the same model in European cities, even small ones, without issues in mixed traffic.
Yup. According to these specs the Metro buses are 2,550 mm wide. The AustRoad guidelines specify “the legal width limit of commercial vehicles is 2.5 m”. The Metro buses had to receive special exemptions to be approved for use on the separated BRT routes they’re going to be running on, and for their more limited testing/promotional phase around the city elsewhere prior to the actual commencement of service (which should be coming in October).