The navy was said to have told the passengers that they were "violating the legal naval blockade", and that if they had humanitarian aid to deliver, there were other ways to get it to Gaza.

A military statement said "the activity ended without exceptional events".

"At this time the ship is being transferred to the port of Ashdod" along the coast of central Israel.

The activists come from Australia, Canada, France, Germany, Israel, Malaysia, New Zealand, Norway, Spain, Sweden, UK and United States.

Gaza's two million inhabitants have been living under the blockade since 2007, when the militant group Hamas took over the area. Activists have tried to break through several times.

The local Freedom Flotilla organiser, Roger Fowler QSM, has written to Gad Popper, the New Zealand Honorary Consul in Tel Aviv requesting assistance, particularly information about Treen's situation and condition; including his health, any medical treatment he may need, any charges he may be facing, and if he has been allowed access to legal assistance

Hehir is calling on Acting Prime Minister Winston Peters to condemn the Israeli actions in the strongest way.

"This was a violent military attack on a civilian vessel and a violation of international law," he said.

"Forcibly taking Mike Treen from international waters to a country which is not their destination is an act of kidnapping, which is also unlawful under the international Convention of the Law of Sea.

"It is interesting that the two Israeli citizens released on bail have been charged with trying to enter Gaza. Not only does Israel not want the world to see what it is doing to the people of Gaza, but it makes it illegal for its own citizens to go there and see for themselves. So much for freedom and democracy in Israel."

Green Party leader Marama Davidson sent a message of solidarity for the trade unionist.

Davidson was in the same situation in 2016 when she was detained by Israeli authorities in international waters near Gaza.

She told Newstalk ZB she felt a sense of unease, not knowing what would happen to her after she was detained.

The Green co-leader recalled thinking that no one would know what happened to them when a boat she was on was boarded in 2016. She hopes Treen and the rest of the crew are released soon.

Only two of the 22 passengers on the ship have been released.