Greater Wellington regional councillors tested out the first electric hybrid prototype at NZ Bus' Newlands depot earlier this year. On-road testing has yet to begin. (File photo)

Wellington could be stuck with its polluting diesel buses for months to come, as a plan to convert the capital's old trolleys into electric hybrids continues to run into trouble.

Major doubt has been cast on the project after NZ Bus admitted it was exploring alternatives to its plan of fitting the retired trolleys with new Wrightspeed motors, which operate mostly on rechargeable electric batteries.

The alternatives range from partnering up with another electric technology company to reverting to diesels instead.

MONIQUE FORD/STUFF A project to install electric hybrid motors on Wellington's retired electric trolleys is looking less and less certain. (File photo)

If the trolleys cannot be reused, they could be consigned to the scrapheap.

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NZ Bus announced in April last year it had signed a $43 million deal to fit a large number of its 1100 buses in Wellington and Auckland – including the capital's 60 trolleys – with Wrightspeed motors.

MONIQUE FORD/STUFF From left, Wrightspeed mechanical engineer Syste Tacoma, NZ Bus chief executive Zane Fulljames, and NZ Bus project manager Sudhakar Arumalla with one of the first trolley buses to be pulled off the wires. (File photo)

But the project has been marred by uncertainty, with the trolley buses already given a four-month extension earlier this year because the Wrightspeed technology was still under development.

The trolleys were eventually retired at the end of October, with surplus diesel buses from Auckland sent south to replace them in the meantime.

An email from NZ Bus chairman Kevin Baker to Greater Wellington regional councillors, obtained by Stuff, reveals a final decision on whether to proceed with the project will be made only once on-road testing with a second motor design is completed early next year.

N/A It had been hoped the Wrightspeeds would be ready to replace the trolleys when they were retired in October, but an estimated rollout date is now undetermined. (File photo)

The email also reveals concerns about whether Wrightspeed has the necessary supply chain and infrastructure to support the technology.

NZ Bus chief executive Zane Fulljames confirmed the first gearbox delivered by California-based Wrightspeed recently was not suitable, and the company was waiting on an "updated" version to arrive.

That would need to be tested first by Wrightspeed, then by NZ Bus, on a prototype bus before on-road testing could begin.

STUFF NZ Bus says it is waiting on Wrightspeed to deliver an updated gearbox, after the first one was deemed unsuitable.

Fulljames played down the setback, saying it was all part of the testing process, but admitted there was growing uncertainty over the project.

"[Wrightspeed has] spent tens of millions of dollars on this process, and we'd be delighted if that's the one we end up running with, but we have other alternatives to explore."

He could not confirm what other companies were being looked at, but they "certainly have some fair alternatives".

STUFF Greater Wellington Regional Council sustainable transport committee chairwoman Barbara Donaldson says she's still hopeful the project will be a success.

"At the moment, we have a contractual obligation with Wrightspeed, and we'll support them until such time as it either succeeds and we place orders, or it doesn't and we choose to part ways."

Wrightspeed was a "solid option" at the time the contract was signed, but other technology companies around the world had since caught up, Fulljames said.

It was initially hoped the Wrightspeeds would be introduced in November last year. The latest target was July next year, when new bus routes and contracts come into effect, but Fulljames said a target date was now undecided.

Greater Wellington Regional Council's sustainable transport committee chairwoman Barbara Donaldson said she was not overly concerned whether the Wrightspeed project came off.

"It's always been the agreement that they [NZ Bus] have to provide the buses.

"If they can get electric buses or electric hybrids through Wrightspeed or any other way, that will be fantastic. If they can't, then they need to be meeting the standards for the Euro 5s or 6s [the most environmentally friendly diesels]."

Fair and Intelligent Transport Wellington (Fit Wellington) spokesman John Rankin said he was disappointed but not surprised by the situation.

"Over the last year or so, there's been a lot of talk about new and emerging technology that we can use in Wellington.

"Those of us who are more conservative were saying, 'Don't pioneer some of this stuff' because if it works, great, but if it doesn't work, what's your fallback position?"

The project could be a lesson for the regional council in being better risk managers, but NZ Bus should be applauded for "giving it a go", Rankin said.

Regional councillor Sue Kedgley reiterated her view that the trolley buses should have been retained until the Wrightspeed technology was proven.

"As it is, NZ Bus still doesn't know whether the Wrightspeed technology will work, and if it did, whether Wrightspeed would even have the supply chain or infrastructure to manufacture, install or support the technology.

"So it looks like it's still a fantasy at this stage."