It was not yet clear how water had got in.
It may have entered through a failed seam in the moulded plastic unit, or the dock may have become jammed on the bottom during one of the past week's unusually low tides. That could have allowed the rising tide to enter around bolts in the top of the dock.
A barge due in the Bay of Islands today for another job would be used to raise and remove the dock.
No one was using the dock when it sank, Galbraith said.
The sinking was ''not a great thing'' but the timing — when the wharf was not in use due to Covid alert level 4 — was as good as could be.