From SH3, take Kent Street to Egmont Street to Mana Bay, which sits on the mouth of the Pātea River. There are public facilities, but the real attraction is the dilapidated north moles, surrounding cliffs, wave surges and, even by the standard of this coast, huge piles of driftwood.
Just to the right of the north mole and across the first pile at about the high tide line is the wreck of the steamship Waitangi from the early 1920s. Apparently it does get covered in sand from time to time, so hopefully it will be visible if you visit.
There is also a parking area on the top of the cliffs to the northwest. This provides a particularly good view of the beach and cliffs to the northwest, as well as an access track down to that section of the beach.
If you have had enough of beaches, take Portland Quay from SH3 immediately before the Pātea River bridge (southside), then the first right over the railway track, past the piles of gravel to one of NZ’s few industrial ruins. It was damaged by fire many years ago and has been left to gradually rot away.