A lost and highly venomous sea snake has been found in Auckland's Viaduct Harbour after it was swept away from the Pacific.

The banded sea snake is a rare sight in Aotearoa, but is still considered a native species as it naturally arrives in our waters via ocean currents.

Department of Conservation spokesperson Clinton Duffy says snakes show up on New Zealand's shores once or twice a year, but this case was particularly unusual.

"Banded sea snakes aren't oceanic species, they generally live close to shore.

"This one's obviously been swept away by ocean currents."

The department's policy is to let it be.

"They're a protected species.

"You're not allowed to harm them or harass them, and they're highly venomous, so the best course of action is to just leave them alone."

Duffy says the snake, which was spotted on Tuesday, is especially venomous, perfect for hunting fish.

"They have a very, very powerful neurotoxin,.

"They are extremely venomous, more venomous than a cobra, so you don't want to be bitten by one."

But the likelihood of being bitten is extremely low, due to the species' unique biology and chilled-out attitude.

"Their fangs are tiny and mounted in the back of their jaw.

"You'd actually have to work very hard to get bitten by one, they really have to work for it."

Although he had never seen one in New Zealand, Duffy said he had encountered them before.

"I've dived with them in the tropics.

"They're not aggressive at all."

As rare as sea snakes are, Duffy says they could become a regular visitor.

"We might start to see snakes [in the water] more frequently.

"It's possible that as the water warms up and they're able to survive better on it, we may see less washing up on the beach."

But it'll be a long time before snakes can thrive in New Zealand.

Duffy says the banded snake seen yesterday is unlikely to survive.

"It's been swept away from its normal habitat, there's very little chance it'll be able to make it back there alive.

"It may survive the summer but as water temperatures drop it'll probably die."

He says holidaymakers are very unlikely to encounter a snake, but reiterated one bit of advice if they did: "leave them alone".

-Felix Walton/RNZ.