A Chinese "spy ship" slipped into Auckland over the weekend, sparking comments on social media from Kiwis feeling uneasy about its presence.
The Yuan Wang 5, listed on the port website as planning a stay from October 2 to 6, is described on Wikipedia as one of a class of Chinese naval vessels "used for tracking and support of satellite and intercontinental ballistic missiles".
Security analyst Dr Paul Buchanan said ships of the same class had visited Auckland since at least 2005.
"They are spy ships," he said. "They are dual platforms. They are used to track satellites, and they are basically intelligence collection platforms. From what I understand, 60 to 70 per cent of their work is looking for other people's signals and 30 per cent is the satellite work."
Aucklander Brook Turner posted a photo on Facebook, saying: "Not sure how I feel about having a boat like this in NZ."
Jayden Dunn commented: "So we throw a hissy over America coming here (and rightly so) but no one even heard about a military class surveillance vessel from PRC [People's Republic of China]? I don't want either of them in our waters. I thought we were a peaceful country with particular military values. I might be wrong."
Buchanan said New Zealand authorities were aware of what the ship was doing and had no reason to stop it visiting this country.
"Part of the reason that it's not problematic is that the United States, as well as other Western nations, also have signals collection ships in the South Pacific," he said.
"Our signals people listen in to them listening to us, that's the way the game is played. There's no compelling reason to prohibit them from coming in."
He said the US and China used their signals collection ships partly to track rival submarines patrolling in the South Pacific. China was increasing its patrols as part of a "three island chains" strategy that sees islands stretching from eastern Russia south to New Zealand as a "second chain" or buffer zone outside an inner island chain closer to China.
"They would rather fight adversaries in the second island chain rather than the first, for obvious reasons," he said.
"So they have to show a naval capacity that will allow them to conduct sustained operations in that second island chain."
He said it was ironic that Chinese tracking ships called into New Zealand ports routinely, yet no US military vessels had visited since New Zealand's ban on nuclear-armed ships was imposed in the 1980s.
That is about to change with a non-nuclear-armed US military vessel expected to visit next month for the 75th birthday of the Royal NZ Navy. Buchanan said that visit would open the gate to more in future.
""This ceremonial visit may be the opening to actual patrol visits, and there would be no reason to refuse them," he said.
A Chinese Embassy spokesman said the embassy was marking a Chinese national holiday and no one was available to comment.