Twice my daughter has come down from Brisbane and taken her kids to Taronga and on neither occasion were they able to see the gorillas. (She jokes that they don't really have any!) On one of those occasions they couldn't even get near the enclosure - it was barricaded off.
That sounds like just unfortunate timing really.
We visit Taronga at least 5 - 6 times each year and have never had trouble seeing the Gorillas. Not that this means terribly much I know.