An old trolley bus has been roaming Wellington's streets with a brand new fully-electric engine.

Wellington's retired trolley buses could be back on the streets by the end of the year after being fitted with fully-electric motors.

The capital's 60 electric trolley buses were taken out of service in October after Greater Wellington Regional Council deemed the $50 million cost of upgrading and maintaining the ageing overhead wire network too expensive.

The trolleys were subsequently replaced with surplus diesel buses from Auckland after their owner, NZ Bus, ran into problems trying to fit them with Wrightspeed motors – an experimental electric-diesel hybrid technology being developed in the United States.

ROGER BLAKELEY/SUPPLIED NZ Bus had hoped to fit its 60 trolley buses with hybrid Wrightspeed motors, but the project hit several stumbling blocks.

It has now been confirmed the company has partnered with China-based Times Electric Group (TEG), which is the same company providing the engines for Tranzit's new fully-electric double-decker buses that will hit Wellington's streets in July.

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The technology appears to be working, with one of the old trolleys already roaming the streets of Wellington on a trial basis.

SUPPLIED Greater Wellington regional councillor Sue Kedgley said NZ Bus should have gone with a more assured option in the first place, which would have prevented diesel buses being used in the meantime.

But regional councillor Sue Kedgley said that begged the question of why NZ Bus toyed with unproven technology in the first place, rather than opting for an established company like TEG, which was founded in 1992.

That could have prevented diesel buses being used since November, she said.

"We were told the Wrightspeeds were going to be ready last August. But it became quite obvious after about three months they were never going to eventuate and I couldn't understand why NZ Bus or even our own officers seemed to be convinced otherwise."

MAARTEN HOLL/STUFF There is no guarantee the refitted trolleys will stay in Wellington.

Wrightspeed, based in San Francisco, was a "small start-up business" that did not even have bus manufacturing facilities, Kedgley said.

"It was quite obvious from the outset it was incredibly new, experimental technology that's never been used anywhere in the world. We were told upfront they had a couple of rubbish trucks in California and that was it."

The council should have stepped in and demanded NZ Bus prove the technology would work, and set a timeline for delivery, she said.

But the council's sustainable transport committee chairwoman, Barbara Donaldson, said it was up to NZ Bus to decide what it did with the trolleys, as long as it met contract requirements.

From July, all diesel buses in Wellington will need to have at least a Euro 4 emission standard for existing operators, and Euro 5 or 6 – the highest global emission standard – for new operators.

"This was entirely a NZ Bus decision to try to refit the old trolleys with some sort of electric system," Donaldson said.

"We certainly supported them along the way."

Retaining the trolley network would have been too expensive, and would not have worked with new routes coming into effect from July, she said.

The committee's deputy chairman, Daran Ponter, said if the trial was successful, the rest of the buses could be rolled out by the end of the year.

But there was no guarantee they would be in Wellington, with the possibility NZ Bus could take them elsewhere.

"So that's a real challenge for us, to grab hold of them. We want to retain these electric buses in Wellington."

NZ Bus chief executive Zane Fulljames could not be reached for comment. He has previously said the company would no longer be responding to media requests.