I think the next phase will be building the railway across to the shore which would tie into that rail loop
I see where you're coming from but I don't think it is necessary. The busway works really well at the moment, even better since the addition of more double deckers. As for the future, simply add more buses to the network as the demand increases.
The problem is at the terminus in the CBD. Buses just can't scale. Even with the HOP card speeding up boarding (though almost 25% still pay cash and slow everyone down) a bus with 60 people can't load / unload as quickly as a train can load and unload 1,000 people. Buses need roads in the CBD and places to stop / dwell. A train wouldn't need any space above ground at all.
It takes years to build this stuff. The worst time to start is 2 years after it became obvious you should have started a decade ago......though that is how NZ typically does infrastructure.