Those few labs that are still processing E-6 have made a commitment to maintaining their chemistry etc and, as professional labs, stand or fall by the quality of there work.
Anecdotally, the two labs here in Melbourne processing E-6 seem to be running good volumes, which helps to maintain the stability of their chemistry. I would imagine that the situation is similar for those in Sydney.
If you have not used transparency film before, there is a definitely a learning curve in determining “correct” exposure and getting to know the contrast latitude of the film, particularly with Velvia RVP 50. I can speak from personal experience that you are far more likely to ruin a sheet/s of Velvia through user error rather than the lab botching the processing.
As Walter has suggested, it pays to do some homework before you embark on a critical shoot with a particular film and shooting an extra sheet or two at the time doesn’t hurt either (except, perhaps, your wallet).
I’m sure any of the labs mentioned would be happy to have to chat to you about your concerns too.