I got such a surprise the other day. The dam has been drying out - we've been trying to kill the wrigglers. I thought there were no tadpoles in it; hadn't seen any eggs and wasn't able to see any tadpoles swimming around either.
I went out to the sump in the bottom of the dam and there were tadpoles rising up for gulps of air. We fished at least 100 of them out with buckets and a flyscreen, and put them in the glyphosate container on the back verandah. Most of them are huge! With a few smaller ones in amongst them. Judging by the size, I think they're probably Mallee Spadefoot Toad (ie. they are that big because the eggs were laid a while ago, when the Spadefoots were calling)**. There is the odd darker tadpole which I think must be Spotted Marsh Frog (lower photo).
We're going to have to rationalise the population a bit - have sent some into school, and are considering whether to introduce some to other dams and ponds around the place.
** have since found out that Mallee Spadefoot Toad tadpoles are dark/black, which rules these out. They could be Common Spadefoot Toad (Neobatrachus sudelli) perhaps?