Mosquito Bay is very much part of the Kaipara system, at the tip of the southern sandbar that encloses and shelters the Kawau Parua Inlet. As it turns out, the sandbar is not a sandbar but a delta, the geological differences being quite important, as the future changes to the area depend on the way in which the formation is taking place. GeoTrips gives a very good explanation. 

The short version is that the lovely sand and interesting sedimentary rocks here have drifted all the way from the ancient Taranaki Volcanoes, creating what is known as a 'flood tide delta'. Over the last million or two years. The flat floor of the harbour here is from older Kaipara Volcanoes. And the happy result is the fishing is reportedly marvellous, there are snapper galore and some fish can be caught by surfcasting. Be aware there have been many shipwrecks on the Kaipara Bar.