Ray's Rest Camping Reserve is a free campground at 562 E Coast Rd, on the Firth of Thames about 4km south of Kaiaua, with a Google rating of 4.6.
It is a long, flat strip beside the sea with views across the firth to Coromandel, and sunrises and sunsets that campers like. You must be self-contained, stays are limited to two nights, and there are no dogs and no fires. The water is shallow for swimming and sharp underfoot for paddling, but flocks of birds feed in the shallows at low tide, and people with boats go fishing. The camp sits on the Hauraki Rail Trail, so you can walk or ride (e-bikes 300W or less) to the fish and chip shop in Kaiaua, or to the Pūkorokoro–Miranda Shorebird Centre, about a 25-minute walk away.
For a quiet stop on the Firth of Thames coast, Ray's Rest suits self-contained campers who want sea views and birdlife.