Updated 8.52pm: Two New Zealand journalists have been detained by the Israeli military.

Maori Television's Ruwani Perera and camera operator Jacob Bryant are among 19 people who were on a ship observing an attempt to take supplies to Palestinian territories.

The flotilla carrying pro-Palestinian activists were seeking to reach Gaza to highlight the Israeli blockade of the territory, with the attempt coming five years after a similar bid ended in a deadly raid.

Three of the boats were said to have turned back while a fourth, the Marianne of Gothenburg, was boarded by the Israeli navy and was being escorted to an Israeli port.

Prime Minister Benjamin Netanyahu insists his government was right to take action against Hamas, the Islamist movement that rules Gaza.

"In accordance with international law, the Israeli navy advised the vessel several times to change course," the military said in a statement.

"Following their refusal the navy visited and searched the vessel in international waters in order to prevent their intended breach of the maritime blockade of the Gaza Strip.

"The forces have reported that use of force was unnecessary, and that the process was uneventful," it added. "The vessel is currently being escorted to Ashdod port and is expected to arrive within 12-24 hours."

Kia Ora Gaza spokesman Roger Fowler said the ship has been intercepted by Israeli forces and is being towed towards Israel.

"We understand from news reports out of the region that the boarding was quite quick and reasonably peaceful."

Roger Fowler has written an open letter to Foreign Minister Murray McCully "and asked him to register protest with the Israeli authorities and ask them to release the boat and the passengers and allow them safe passage to their destination."

MFAT released a statement saying it told the journalists to heed travel advice, which advises against all travel to Gaza due to the threat from kidnapping, acts of terrorism and potential for retaliatory military operations.

It also strongly advises against any attempt to enter Gaza by sea, including participation in flotillas to deliver aid.

Roger Fowler said they haven't been able to make contact with the pair.

The other three ships in the flotilla had changed their course and were "heading back to their ports of origin," according to a statement by "Canadian Boat to Gaza," issued by the activists before the Israeli navy commandeered the Marianne.

"We once again call on the government of Israel to finally lift the blockade on Gaza," the statement read. "Our destination remains the conscience of humanity."

Netanyahu dismissed the organiser's goals.

"This flotilla is nothing but a demonstration of hypocrisy and lies that is only assisting the Hamas terrorist organisation and ignores all of the horrors in our region," he said in a statement.

"Preventing entry by sea was done in accordance with international law and even received backing from a committee of the UN secretary general."

Defence Minister Moshe Yaalon said the flotilla "wasn't humanitarian and didn't seek to help anyone," adding that "the participants were seeking to continue the campaign to delegitimise Israel."

Israel imposed its blockade on Gaza in 2006 after Hamas captured an Israeli soldier, and tightened it a year later when the Islamist movement consolidated control of the territory.