What is this?

[POLAR GOULDIANS]

Hi Cranberry,
As Matt has said, the main sign for Aust Yellow is having pearly white nodules instead of blue nodules, if the young birds don't have these they are definitely not Aust Yellow.
As Dano has said the Gouldian hens are prone to mating outside the nesting pair, they are also very prone to laying eggs in other pairs nests. So there are quite a few ways to get strange results.
Normally an Aust Yellow is a green bird with the two recessive ( Aust Yellow ) genes attached, making it appear as an Aust Yellow. However you may have a sex linked yellow ( SF or DF ) with the two recessive genes attached, this bird would appear as a Aust Yellow, but when bred with normal hens would produce young consistant with being a sex linked yellow.
If this was the case you may also get SF sex linked yellow cocks as well.
Going back to the start though, if the young bird has blue nodules, then it is a sex linked yellow.
I tend to agree with Matt that one of your yellow cock birds is actually a sex linked yellow.

[Tiaris]

Me 2.