Hello Ross, not sure about this species, I'm not familiar with Gerygone mouki but from what I can see G. mouki has a clearly defined white line above the eye, which this bird doesn't appear to have, maybe someone else might have a better idea of this species.
Yes, I think I have heard that but was undure about this one. Had a similar sighting on Twofold Bay and it was thought to be a female or immature Pachycephala pectoralis (Golden Whistler).