Local civil rights lawyers met with all those detained a few hours ago.
Treen said all their belongings, including their passports, have been seized and not returned, despite promises they would if they co-operated.
The Unite Union is subsequently asking for the NZ Government to condemn Israel over what it calls a violent attack and unlawful detention of its national director.
"Despite KiaOra Gaza contacting the NZ Hon Consul in Tel Aviv requesting assistance we are unaware of any contact, visit or action from the NZ Government or its representatives to assist," Unite national secretary Gerard Hehir said.
The incident took place on Monday morning, while the crew of international campaigners were taking part in a mission to deliver aid to the Palestinian territory of Gaza. The boat was said to have been loaded with medicines.
Eyewitnesses report that Treen's boat was hijacked in a violent raid and that he and the rest of the crew are being held in Givon prison, 100km north of Tel Aviv.
Only two of the 22 passengers on the ship have been released.
One of those released, Zohar Chamberlain Regev, the boat leader, reports that at the time of boarding: "People on board were tasered and hit by masked IOF soldiers.
"We did not get our passports or belongings before we got off the boat. Do not believe reports of peaceful interception," he said.
Regev also reported seeing blood on the deck of the Al Awda as the last participants were being dragged off the ship.
Hehir said they were awaiting information about Treen's condition, charges he may face and what the Israelis intend to do with the remaining 20 flotilla participants they have imprisoned.
Hehir says Treen will probably be deported after a few days, but it's not without risk.
The Israeli Navy was quoted as saying that it had stopped a ship of activists trying to break its blockade of the Gaza Strip.
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 Embassy of Israel in Wellington said the flotilla to Gaza was in direct violation of international law.
"[It] is a cynical publicity stunt designed to divert the attention from the fact that Hamas, with its brutal ruling of Gaza since 2007, invested in terror instead of the welfare of its people," it said.
"The naval blockade is legal and in accordance with international law. By imposing the legal naval blockade, Israel is exercising its right to self-defense by aiming to prevent the illegal smuggling of arms to Hamas, a world-recognised terror organization [including by New Zealand] that is sworn to Israel's destruction and had turned Gaza into a terror center."
The embassy said the flotilla to Gaza would not promote peace or assist the Palestinian people.
"It will only inflame the situation in the region. In fact, humanitarian aid is already provided to Gaza through Israel in vast quantities making the boats unnecessary.
"This cynical public relations stunt, endorsed by extremist groups such as Kia Ora Gaza, is first and foremost motivated by hatred and anti-Semitism and does not contribute to the promotion of a possible constructive engagement between the parties."
The activists come from Australia, Canada, France, Germany, Israel, Malaysia, New Zealand, Norway, Spain, Sweden, UK and the 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.
A spokesman for The Ministry of Foreign Affairs and Trade said it continues to advise against all travel to Gaza.
"This includes any attempt to enter Gaza by sea in breach of Israeli navy restrictions or participating in any attempt to break the naval blockade," it said.
