The mystery surrounding a French warship found by Coastguard “lurking” in the Hauraki Gulf appears to have been solved, with the French Embassy providing fresh details today on the frigate’s surprise discovery.

Radio silence from authorities and embassy officials about why the heavily armed navy vessel was located earlier this month within New Zealand’s 12 nautical mile territorial limit sparked speculation from a defence expert that it may have been hunting Chinese submarines operating near New Zealand waters.

However, the French Embassy says FS Vendémiaire was simply conducting sea trials after undergoing maintenance in Auckland and that its presence in domestic waters had been notified to New Zealand authorities.

Prime Minister Chris Hipkins was blindsided today when the Herald asked him what the ship - which is fitted with a 100mm artillery turret, and two mounted 20mm guns - was doing off Auckland’s coast.

Stunned Coastguard crew came across the ship during the search for a stricken yacht. The French frigate responded to the mayday call and used its radar to help locate the distressed vessel.

The 94-metre frigate is not nuclear-powered and does not carry nuclear weapons. It is part of the French Pacific fleet and is usually based in New Caledonia.

The New Zealand Defence Force [NZDF] has refused to answer questions about the nature of the ship’s visit, directing inquiries to the French Embassy, which has remained silent until today.

Hipkins said he had no knowledge of the ship’s visit during a media standup this morning. His office later provided a statement saying the FS Vendémiaire regularly visited New Zealand, and that the latest visit “was done with the full knowledge of the NZDF and was approved by MFAT”.

Hipkins’ office said the French ship was not involved in any joint exercises, “but for specifics about the Vendémiaire’s programme you would need to speak with the French Embassy”.

Stunned Coastguard crew discovered the French warship in the Hauraki Gulf during the search for a stricken yacht. Photo / File

After questions from the Herald, the embassy released a statement this afternoon saying the ship was completing sea trials after two months of maintenance at a Devonport shipyard before returning to her home port in Noumea.

“Like for any port visit, this long presence in New Zealand has been notified to the New Zealand authorities,” deputy press attaché Clarisse Christie said.

“The presence of FS Vendémiaire in the Hauraki Gulf was normal as the gulf is in front of Auckland.”

Christie said Coastguard crew “may have been surprised” to encounter a warship in the area, but the frigate’s actions in responding to the mayday call helped identify a weak but sufficient radar echo on the distressed boat.

“Safeguarding life at sea is part of the DNA of every single seaman and FS Vendémiaire is proud of having been able to help in this difficult operation. She has completed her sea trials and has left New Zealand.”

Paul Buchanan is a former intelligence and defence policy analyst and director of the 36th Parallel Assessments.

He said the veil of silence about the ship’s visit from government agencies was unusual.

France was a “friendly country”, Buchanan said, but could not enter our territorial limits without permission from the government.

He said foreign warships sometimes conducted anti-submarine operations off New Zealand’s east coast in tandem with “attack subs”, usually targeting Chinese submarines which had been known to operate in the area.

Security expert Paul Buchanan says foreign warships sometimes conduct anti-submarine operations off New Zealand's coast. Photo / File

Absent any official explanation for the French warship discovery, he said this was an equally plausible reason for the FS Vendémiaire’s visit.

“Destroyers are often used to hunt submarines but in the modern day that’s done in tandem with attack submarines. We can’t hunt them because we don’t have the resources. The best way to do that is to have a submarine quietly listening for them coming through.”

Howerver, as our ally, it was possible the French ship was simply involved in normal operations deployments with permission from the New Zealand government, meaning domestic government agencies would have knowledge of the ship’s whereabouts and mission, Buchanan said.

“What is unusual about this is the NZDF’s reluctance to give more detailed information about what [the warship] is doing.

“It’s really odd.

“I would argue that the NZDF needs to be a little more transparent about the nature of this visit.”

Stuff reported that the ship last visited in 2009 when it conducted a joint training exercise with the New Zealand Navy as well as diplomatic duties.

It is designed as a surveillance vessel, patrolling French overseas maritime areas and its exclusive economic zone.