UK foreign secretary Boris Johnson had an eventful visit to Zealandia wildlife sanctuary on Tuesday, congratulating the Government on its "campaign of slaughter" and comparing a bird's loss of wings to nuclear disarmament.

Johnson, in the country on a two-day jaunt in between trips to Japan and Australia, visited the Wellington wildlife sanctuary as a guest of Minister of Culture and Heritage Maggie Barry, on the one-year anniversary of Barry's pledge to make New Zealand predator-free by 2050.

On a brief walk through the sunny woods Johnson was treated to sightings of a kereru, a kaka and a tui - all native birds the sanctuary is dedicated to saving.

NZN VIDEO British foreign secretary Boris Johnson enjoys joke-filled tour to Zealandia wildlife sanctuary in Wellington.

The critically endangered takahe - named "T2" (after the film) and one of just 300 or so alive - prompted Johnson's nuclear comments, after his guide advised that the species had evolved to lose its wings after arriving in New Zealand.

READ MORE:

* Boris Johnson comes bearing laughs, but what about gifts?

* 'Predator-free' by 2050

* British Foreign Secretary Boris Johnson pays tribute to UK-NZ bond

"I don't want to be disobliging to the takahe, but it's not surprising given that they dispensed with their wings, that they are now finding life tough. It's a bit of a lesson for us all isn't it?

CAMERON BURNELL/POOL Maggie Barry attempts to point out a bird to Boris Johnson.

"It's like you know nuclear weapons, if you disarm..." he trailed off, as his guide said he wouldn't want to say anything "political" himself.

"You're quite right, they are fast runners though," Barry replied.

Johnson knelt down to take a photo of the bird on his phone before moving on to a weta cage held within a tree.

CAMERON BURNELL/POOL British Foreign Secretary Boris Johnson holds a tuatara or "tuatora" as he repeatedly called the reptile.

"Wetapungas can be the size of your hand - the giant weta," Barry noted.

"Is that a giant weta?" asked Johnson.

"No, this is a tree weta," replied a guide, "that's why it's in a tree."

CAMERON BURNELL/POOL Boris Johnson snaps a pic of a Takahe named "T-2".

"A big weta will take your whole hand - well, they draw blood," Barry said.

The entourage stopped to see a kereru perched in a tree above. Barry explained that the birds sometimes eat so many berries they lose the ability to fly, even falling to the ground and bursting.

"That's a sort of anti-obesity campaign. You should run with that one," replied an astonished Johnson.

The trip began and ended with brief encounters with tuatara - which Johnson noted had a "marmoreal indifference" to him.

The tuatara resembles a lizard but is in fact the last example of the Sphenodontia order of reptiles, most of which died off over 60 million years ago.

They were considered "living fossils" for many years but scientists now believe some of their cell structure has changed over the last 200 millions years.

This prompted some comments from Johnson as he held a tuatara, which he repeatedly called a "tuatora".

"It's a 200 million-year-old life form! Absolutely beautiful," he said.

"He's hyperventilating with excitement," Barry said, as the tuatara looked on at the assorted media pack snapping away.

After a discussion about whether the tuatara was still evolving or not, Johnson openly wondered whether humans were still evolving too - possibly into lizards.

"There's a chap in Britain called David [Icke] who thinks that the world is in fact ruled by people like this," Johnson said of the tuatara.

On their way out the pair discussed New Zealand's campaign to become predator-free by 2050, which Johnson described as a "campaign of slaughter".

"Thank you very much for having me here on this important anniversary day of your campaign of slaughter of rats, and possums, and stoats! And other animals," Johnson said.

"I don't think people actually grasp the threat to birds that these little mammals can pose, and I'm sure what you are doing is done in an entirely humane and dignified way, and they die with a smile on their faces, and I congratulate you on that."

Barry said Johnson could ask for DoC expertise back home if needed.

"If you have a problem with your grey squirrels just let DoC know. We can come and help out," she said.

"You'll come and programme a painless extermination," Johnson replied.

Asked by media to pick a favourite creature, Johnson said it was a split between the "tuatora" and the takahe, despite its flightless and thus nuclear-free status.