A devout Israeli tourist aboard the Interisland ferry Kaitaki sparked a bomb scare and police armed offenders' alert today after the ship's captain spotted boxes and what looked like wires taped to the man.

"One individual had two boxes attached, one box taped to his leg and one box seemingly taped to his forehead," Kiwirail spokesman Kevin Ramshaw told NZPA.

Jewish websites describe a traditional prayer ritual where a small black leather box called a "tefillin" containing verses from the bible is taped to the arm and forehead.

"These may well have been part of religious observance, but to people who are involved in the travel business, there were what seemed to be wires attached to them."

Mr Ramshaw said the captain then followed normal procedure by notifying police of his suspicions, sparking an armed offenders' alert in Picton.

Crew then spent a nervous three hours closely observing the man as they sailed the Cook Strait, to avoid mass panic.

"Staff felt the better thing to do was observe and stay away."

Mr Ramshaw said the 750 passengers aboard the ferry on the Wellington-Picton trip were "probably" not aware of the perceived threat.

Police spoke to the Israeli tourist and his three travelling companions upon arrival at Picton, before releasing them without charge.

"The guys were, in the eyes of people, acting suspicious, but in actual fact they weren't," Senior Sergeant Peter Payne told NZPA. "I believe that they were just going to pray or doing something like that."

Reports the captain had been taken hostage were inaccurate.

"There is no hostage, there is nothing like that at all, no explosives, no firearms or anything ... They haven't done anything wrong at all."

Southern Communications Inspector Paul Fremaux said police were obliged to take such reports seriously, particularly when they involved members of the public.

"The captain spoke to us and he was concerned and I guess anyone ... who is charge of any transport with members of the public, you've got to take it reasonably seriously, and we did, until proven otherwise."

In January this year, a plane flying from New York to Kentucky was diverted to Philadelphia after a 17-year-old boy using a tefillin to pray alarmed passengers and crew, who feared a bomb scare. He was released without charge.

Mr Ramshaw said police told him the tourists were very understanding of their detention and came from a military background in Israel.