The same as on Mothers day, Perth Zoo is giving Free Entry to Dad's on Father's Day - the 6th September. I assume other zoo's in the group will also be doing the same.
I couldn't find it on their webpage, but came up on their Facebook page.
Doesn't help me much as we have annual passes, but well worth a day out, especially if the weather is cool as the animals come out more.
Do they check if you're really the dad?