First British Navy ship in more than 20 years berths at Auckland's Devonport Navy Base

Forty million items of plastic, on one of the world's smallest and remote islands.

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It’s a sight not seen in more than a quarter of a century. Source: 1 NEWS

It's a sight which is hard to fathom, but it's one encountered by crew on a British warship on its way to New Zealand.

HMS Montrose today became the first Royal Navy warship in more than 20 years to berth at the Devonport Navy base in Auckland.

While its crew is trained for battle, they've been helping record information in another kind of battle entirely – plastic in the Pacific Ocean.

Henderson Island in the Pitcairn Island group was one of their remote targets.

"It's really tragic as a mariner to see just how much plastic waste has washed up on these otherwise pristine beaches," Commanding Officer Conor O'Neill told 1 NEWS on arrival into Auckland.

"Some of it's fishing gear... it's more plastic barrels, plastic chairs, grills, netting - all sorts of plastic waste ... which has clearly been dumped in the ocean," he said.

The more than 200 people on board were greeted by an impromptu haka, and Commander O'Neill said he'd never witnessed anything like it.

"I've never been greeted in quite such spectacular style,' he told 1 NEWS.

Also on board was the ship's sole New Zealander, Petty Officer Heidi Tait, who left for Britain almost a decade ago on her OE, joined the navy and stayed for her new life.

She hasn't been home to New Zealand in about two years.

"I saw there were a lot of people on the jetty and I didn't really know what to expect at all, and then they started doing the haka and it was incredibly dramatic, absolutely fantastic.

"What was amazing was just how much our ship's company was buzzing as we came off the upper deck, they were so excited it was absolutely tremendous," she told 1 NEWS.

We're really lucky, we've passed some unusual places, Pitcairn Island ... Easter Island, that's somewhere people don't often get the chance to see," Petty Officer Tait said.

The British crew will now take in the likes of Hobbiton near Matamata, and perhaps Rotorua, before heading off.