20 x 17 x 7.5
has been attacked by borer

My ID not certain, by :-
Limpets guide, Patelloida corticata looks likely, then Google search images, ->

fig. 9 side view (N) Notoacmea cellanoides not Patelloida (Z)inhttp://allanwilsoncentre.ac.nz/massey/fms/AWC/documents/Publication/Nakano%20et%20al%20MollRes%202009.pdf

• nice photos, id by DNA

then side and top view in http://collections.tepapa.govt.nz/taxon/16515
top view also shows thickening on one side (left if long end is front)

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Tag "Orewa 2016" June photo 4
Also paddle crab, sand dollar
Flapjack, Neptunes necklace
Crabs, 15-20mm green rectangular, under rocks at Red Beach Surf club stream
Other stuff forgotten
Lots more smaller young than commonly seen at NBO