Launceston, about 30m
Fog was forming in all the low spots on the way to the airport about 4:30am.
Tried to get loaded and out asap.
It literally rolled over the airport as we started to taxi from CAVOK to nothing.
As we had already started to vacate the apron we continued to taxi out and could only just make out one centre line taxi way light before we could see the next shining through the fog.
Sat on the threshold for 15+min as the vis varied but never got above 200m til we taxied back in and shut down.
Of course the fog started to clear and a minute or two after shutdown.