Mystery plane lands in Wellington as top-secret Five Eyes conference takes place at Millbrook Estate resort in Central Otago's Arrowtown

Another mysterious aircraft has arrived in the country, as a top-secret spy conference begins.

Fairfax reported last night that a Gulfstream jet with blocked credentials landed at Wellington Airport during the evening.

A screen grab of online aircraft tracker FlightRadar24 showed the flight's arrival, but it was not known why it was in the capital.

The plane's arrival follows a cryptic week of aircraft arrivals in the country, as a Five Eyes spying network conference takes place at luxury Millbrook Resort, near the Central Otago town of Arrowtown.

A United States Air Force C-40, a military version of the Boeing 737-700C, on the tarmac at Queenstown Airport on Monday. Photo / Brett Phibbs

Five Eyes is a global alliance of the United States, the United Kingdom, Canada, Australia and New Zealand.

On Monday, a Boeing C-40 identified by its registration number as belonging to the United States Air Force landed in Queenstown.

Fairfax reported another US government plane also landed in Invercargill yesterday.

Occupants of the planes were not known, but both touched down the morning after FBI director James Comey arrived in Queenstown in a chartered Gulfstream Aerospace plane.

FBI director James Comey arriving in Queenstown on Sunday. Photo / Brett Phibbs

The Gulfstream landed before a CIA jet, which arrived via Wellington and may have contained the agency's director, Mike Pompeo.

Pompeo is among members from the agencies believed to be attending the Five Eyes conference, which the Herald understands began on Monday and will last a week.

Police and security personnel were scattered across the grounds of Millbrook Resort during the weekend, keeping a close eye on the comings and goings. Locals reported seeing snipers and bodyguards in Arrowtown last week.

Prime Minister Bill English said he would meet those involved in the conference and reaffirm the Five Eyes relationship, and its importance to the security of New Zealanders and people around the world.

Asked if there was any suggestion New Zealand was not pulling its weight inside the alliance, English said there may have been issues in the past, but no longer.