They never announce that the band is really a bunch of studio musicians backing the singer, or whatever. Look at Guns -n- Roses when it was basically Axl and some band. Same for Chicago - half the people on the stage at most of their shows weren't even born with CTA came out.
That said, I decided to do some research rather than speculate, and I was right and wrong about the girls. Each of them have retied and re-joined several times. The guitar player, Keith, retired from touring a few years ago. That said, they all join together for high profile performances, like broadcast events and such. I cannot find any info on the touring line-up for the Australian tour.