There's plenty of water in the channels even at low tide for a 6m boat. I'd advise you to take a reccy trip up there in the bright sunlight and have a good look around. The channel from Omokoroa to Pahoia is quite wide until you get just past Pahoia. The other side leading to Flax point is narrower. A 6m boat should be able to cross over the middle sand bar at high tide but be cautious. Stick to the channels for now and only go on the flats on an incoming until you know your way around better.
Good luck.