sibeen said:
Was up at one of my brothers property on boxing day and he had this duck swimming around in his dam.

He says it turns up for around six months every year and then buggers off for the winter. Never has a mate with it. I don’t think I’ve ever seen one before.
Any ideas, braintrust?
In the Australian backyard bird count, that would be classed as “domestic duck”. These are a lot larger than native ducks. Domestic ducks tend to be admixtures of Pekin, khaki Campbell, indian runner with a few other colour variants thrown in. People who keep ducks for their eggs or meat don’t care much about the colour of the ducks and so they frequently end up with really weird colour combinations.
Muscovies tend to breed true, but this one has a ghosting colour around the eye that suggests Muscovy. Muscovies are even larger than domestic ducks.
The bluish tinge of the bill may or may not suggest interbreeding with blue-billed duck. The alternative would be from the hook bill breed.
Here are some pictures of domestic duck breeds.
https://poultrykeeper.com/duck-breeds/