Technically, Pātea Beach is in the Taranaki rather than the Whanganui region, but at a 50-minute drive, it is closer to Whanganui than New Plymouth.
From State Highway 3, 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 huge piles of driftwood, even by the standard of this coast.
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 gets covered in sand from time to time, so hopefully, it will be visible if you visit.
There is also a parking area on top of the cliffs to the northwest. This provides a particularly good view of the beach and cliffs to the northwest and an access track down to that section of the beach.
If you have had enough of beaches, take Portland Quay from State Highway 3 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 rot away gradually.
