My apologies for the poor photos, it took off before I could get the SLR. I thought it might be a Podiceps as it was considerably bigger than the grebes we normally see. It was not much smaller than a Pacific Black duck on the dam at the same time.
The dam doesn't seem to be very big at all, and Great Crested Grebes only really occur on significant large waterbodies. On a small farm dam like this it would have to be an Australasian, especially with a juvenile indicating it has bred in the vicinity. Also doesn't look like a GCG.