What!Do you drive?
I've been twice this year, and will definitely visit again, despite living in the Eastern suburbs too. Its a great zoo to wander around but there is a fairly small collection, so it only takes a half day.
That's why I go to Healesville more often. It takes a similar amount of time to wander around and I can use the other half of the day visit a winery or something. What else is there in Werribee? The drive out to Healesville is nicer too.
And I have a car.