Hamilton should "stand up for its local hero" and suspend ties with its Japanese sister city Saitama while environmentalist Pete Bethune is in custody, according to the Green Party.

But Mayor Bob Simcock has rejected the call, saying the Hamilton City Council was "not a vehicle for political activism".

Mark Servian, the Green Party's Hamilton-based spokesperson, said suspending the city's relationship with Saitama, which is just 25 kilometres from where Mr Bethune is being held, would send a clear message to the Japanese and New Zealand Governments that Hamiltonians are unhappy about how Pete Bethune is being treated.

Mr Bethune has been in custody in Tokyo for close to a month, and faces several charges after boarding the Japanese whaling vessel Shonan Maru II in February. Mr Servian said suspending the sister city relationship – in place for nearly 30 years – would "be only symbolic, but that alone will carry great weight".

"And if the (Hamilton City) Council hype is to be believed the tie has economic value too, so such a move would hopefully prompt Saitama to pressure their Government and send an unequivocal signal to Tokyo on where Hamilton stands on this issue," Mr Servian said.

"I call on local MPs Tim Macindoe and David Bennett to back Hamilton suspending sister city ties with Saitama."

Mr Bethune's father Don said he was aware of the sister city relationship with Saitama, and backed the Greens' call.

"I agree completely that something needs to be done," Don Bethune said. "It is indeed a matter of considerable interest.

"Anything that can be done to focus attention on the need for proper responsible behaviour (from the Japanese) would have my support."

He hoped suspending the sister city relationship would have some effect, and was aware his son's situation was generating discussion in Japan: "There is interest in it, but very little information."

Mr Simcock said the council had never touted an economic value from the sister city relationship with Saitama, and such a benefit had never been identified by the Hamilton business community.

The relationship with Saitama essentially facilitated exchange trips for intermediate-age school students – trips which had given Hamilton children "a cultural experience" over the last 25 years.

"That's the only real activity that occurs. I wouldn't dream of putting that at risk ... to play political games," Mr Simcock said. "I don't think it's the role of local government to get involved in foreign policy."

Saitama's mayor and a group of officials visited Hamilton about four years ago, Mr Simcock said, and the only money spent on the sister city relationships had been "on the odd lunch or dinner for visiting representatives from those cities".

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