The Kaipara Harbour (Wikipedia) is a huge inland harbour of some 940 sq/km north of Auckland.
The southern peninsular is relatively accessible from Auckland, its northern tip (through a private forest road) being about 100km and 1.5 hours from Auckland. The northern part and the Pouto Peninsular in the north is a 3.5 hour 244Km drive.
The southern peninsular is a pleasant drive on sealed road (except at the very northern end) with low traffic volumes through farming country.
Lagoon at northern tip - I was there at low tide just a vast expanse of shallow water and mangroves
A better section of the gravel road through the Woodhill Forest (open to the public) which is the only access to a huge lagoon (above). You don't need a four wheel drive but it would help - there are 5-10 km of this.
Looking south from Shelly Beach
Looking north from Shelly beach, an incredibly popular boat launching area at the weekend and home to one of the best cafes around. You won't get any of that fancy food city food but you'll get outstanding value, good coffee and a pretty decent view - below.
Towards the norther end of the peninsular is one of Auckland's newer Regional Parks "Te Rau Puriri". Unfortunately the entrance is some way from the bay (as above) so you would need to pack in any picnic supplies - I was not able to work out if there were any facilities at the beach but generally speaking I've found all the regional parks to be equipped with toilets. This like most of the other parks is a working farm and (below) it is currently lambing time (we are famous for our high quality wool).