Werribee's incredibly close to the city; technically just a 20 minute drive down the highway. Once Werribee completes their expansion, they'll be in a fair run competing with Melbourne. Especially with the addition of elephants, they'll give Melbourne a run for their money. Even the ongoing expansion of the Western suburbs too (around Werribee) is assisting with their rising visitor numbers over recent years.
But to be honest, I doubt Werribee will ever outsell Melbourne. But they'll definitely come close.
Monarto, on the other hand, is almost an hour away from Adelaide, but may be one of the best open range zoos worldwide in years to come. Still, I know a lot of families that won't visit zoos just due to their distance. Healesville Sanctuary is less than two hours from Melbourne, but dosen't receive even half the visitors Melbourne gets; most are locals, but tourism dosen't really reach there.