LB, you're right, sorry my original crankygram wasn't very clear. At ground level up until approx 9am the Pennant Hills area was socked in. Maybe 300m visibility at best. The fog started to break up around 9am and individual clouds were starting to become distinguishable, albeit below what appeared to be a solid overcast.
The aforementioned PA-28 was low enough. I think the regs state registration markings distinguishable at 150m to the unaided eye? I hope I'm wrong, and old mate was flying above the breaking up fog, but it sure didn't look that way.