Hey Kel,
Crabs seem to run with the tide, sort of getting a free ride along the bottom. In my experience you will probably find the crabs have just moved on as the tide turned, as they usually move in schools. My bet would be to try the same area at the same tide next time and you will probably have the same result. If the tide is opposite, (as if the tide was coming in) I would try the opposite end of the bay as that is the way they would be moving.
Personally I have never crabbed Botany Bay but I have many other places and this I have found out over many years of trial and error.
Hope that helps.
Cheers,
Charmo