Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Squamata
Suborder: Serpentes
Family: Elapidae
Genus: Hydrophis
Species: H. platura
Binomial name: Hydrophis platura
Synonyms: Pelamis platuros, Anguis platura, Hydrus bicolor, Hydrophis platura, Pelamis bicolor, Hydrophis pelamis, Pelamis ornate, Pelamis platurus, Hydrus platurus, Pelamydrus platurus,
Pelamis platura,
Common names: Yellow-bellied sea snake, yellowbelly sea snake, Pelagic sea snake

Hydrophis platurus is a species of snake found in tropical oceanic waters around the world. The yellow-bellied sea snake is one of the most widely distributed snakes in the world. It is completely pelagic and is often observed on oceanic drift lines, using surface currents and storms to move around the ocean. This species has been reported in colder waters such as the coasts of southern California, Tasmania, and New Zealand. It is the most commonly observed sea snake in our waters with over two hundred observations. Most of the recorded sightings of this snake are in the northeast of North Island, however, they are spotted coastally as far south as Cook Strait.

Niwa principal scientist Dr Malcolm Francis says. “The Yellow-bellied sea snake is usually found in warm waters, but regularly washes up on North Island beaches. They live near the surface, hang around with logs and drifting seaweed, and feed on the fish that aggregate in those areas."
While all sea snakes are poisonous, Niwa says they generally require a lot of provocation before they will bite, and as their mouths are small and their fangs are set well back it is hard for them to bite a human.
According to the Department of Conservation, they are considered a native species under the Wildlife Act 1953. This is because they arrive here naturally from time to time on ocean currents (rather than by human transportation). It is, therefore, illegal to kill or harass any sea snake or possess one or any part of one without a permit.
Sea snakes are highly venomous with a very potent neurotoxic venom which it uses to prey on eels and small fish. They are docile creatures and usually only bite in self-defence when accidentally grabbed. There is no record of anyone in New Zealand being bitten. Nevertheless, if you find a sea snake keep well away and call 0800 DOC HOT.

Hydrophis platura the yellow-bellied sea snake, as the name implies, has a distinctive bicolour pattern with a yellow underbelly and brown back, making it easily distinguishable from other sea snake species. The total length for males is up to 720 mm, for females up to 880 mm; the tail length for males is up to 80 mm, and females tails are up to 90 mm in length.
Yellow-bellied sea snakes are fully adapted to living their whole lives at sea: mating, eating and giving birth to live young (ovoviviparous). Adaptations to aquatic life include the reduced ventral scale size, laterally compressed body and paddle-tail for swimming, valved nostrils and palatine seal for excluding seawater, and cutaneous gas exchange for prolonging dive times. This species can uptake up to 33% of its oxygen requirements through the skin while diving and swimming at the surface of the water.
They are highly venomous with a very potent neurotoxic venom which it uses to prey on eels and small fish.

Watch a video of Dr Brian Gill, curator of land vertebrates discusses New Zealand's sea snakes
https://www.youtube.com/embed/Q4PZQBeOsIU 

Hydrophis platura Synonym Pelamis platuros, (2).jpg

A photo of a yellow bellied sea snake in Kelly Tarlton's Aquarium.in Auckland. It has since died and is preserved in the Auckland museum.  
Hydrophis platura Synonym Pelamis platuros, (1).jpg

Thanks to Wikipedia for text and information: https://creativecommons.org/licenses/by-sa/3.0/