What we know, and don't know, about the Wellington mystery mayday call

A mayday call from a boat reportedly taking on water off Wellington sparked a massive search operation on Monday, but almost two days on, nothing has been found.

The mystery has deepened since police announced on Tuesday afternoon the search was being suspended - but they did still want the public to get in touch with any information.

Here's what we do (and don't) know about the mayday call and the subsequent search.

What did the mayday call say?

The mayday call, which came in about 10pm, had been quite short, Wellington Harbour Master Grant Nalder told Morning Report.

In it the person talked about there being two people on board a boat that was taking in water and gave its position, about 3 nautical miles from Karori Rock, Nalder said.

"Usually you'd try to get a bit more info, but there was no reply.

"It came through on harbour radio which in itself was a little unusual, normally a mayday call would come through a different channel."

What did the search entail?

Police and the rescue co-ordination centre ran the search, Nalder said, which began on Monday night.

Seven vessels were involved at its peak, including one of the Interislander ferries, Kaiarahi, which was carrying passengers.

Kaiarahi passenger Tupoki Wairau-Hunter said they were told the sailing would slow down to search as it approached Wellington just after 11pm on Monday.

People went on deck and were encouraged to let the crew know if they spotted anything.

The sailing was delayed by a couple of hours but Wairau-Hunter said people were happy to help and concerned about the fate of the boat's crew.

Nalder said several hours were spent that night looking for the boat and people in distress but nothing was found.

"There were really good conditions on Monday night - very little wind, and it would be surprising not to find any sign with that sort of search going on."

A spokesperson from Seaworks Ltd said its their cable zone patrol ship Sea Patroller was also called in to assist - shortly after the mayday was reported - but was stood down after a little over four hours.

During the search, Nalder said South Coast boat ramps were all checked but nothing of interest was found.

Search and rescue also canvassed the shoreline.

Why was the search suspended?

The decision to suspend the search was made by police on Tuesday afternoon, Nalder said, pending any further information.

"There was nothing to support someone was missing... nothing indicating a boat in the area."

Wellington District Search and Rescue Coordinator Sergeant Jonathan Westrupp said no objects of interest were found in the strait or during searches of the shoreline or boat ramps, and no boaties were reported missing or overdue.

Was the call genuine?

Nalder couldn't say whether the call had been genuine or not but did say it was "unusual" to have such a short mayday call.

He noted it was also odd there was no reply when the call-taker was asked questions.

"It's very hard to take it on that initial call what it would be... it's unusual to get hoax calls... it's hard to say for certain..."

"I would hate to think it was a genuine call and something that did get overlooked.

"But there's significant time for people, fuel and disruption so it's not treated lightly."

Nalder said if the call was not genuine it would be "very disappointing".

"A lot of people were involved, even the passengers on the ferry, and they were genuinely trying to help so it'd be frustrating if it was because someone thought it'd be a bit of fun or something stupid like that."

Westrupp said police were making inquiries into the mayday call to determine its source and veracity.

What happens next?

Until any extra information is received by police the search will remain suspended.

Nalder said if it did turn out the call hadn't been genuine it would be very hard to figure out where it came from.

Sign up for Ngā Pitopito Kōrero, a daily newsletter curated by our editors and delivered straight to your inbox every weekday.