Key Points:

An apparent algal bloom has discoloured the sea around Waiheke Island, in the Hauraki Gulf.

nzherald.co.nz reader Rob Waardenburg sent in a series of photos of the bloom snapped on his way toward the Motuihe Channel from Port Jackson, on the Coromandel Peninsula.

Mr Jackson said he had never seen such a concentration before, with some areas of water coloured crimson.

The Auckland Regional Council said the bloom was most likely to be Noctiluca scintillan, a species which is harmless to humans.

Algal blooms can be quite common during summer due to an abundance of microscopic food, warm temperatures and stable weather conditions.

- NZHERALD STAFF