Old diesel buses on the streets of Wellington have caught the attention of some locals.

Diesel buses up to 20-years-old have been brought into Wellington to cater for growing demand on the capital's public transport network.

The move has disgruntled some Greater Wellington regional councillors, given the buses fall below the emissions standards the council expects all buses to adhere to from the middle of next year.

Some social media users expressed curiosity after spotting the grey buses cruising the streets of Wellington alongside the traditional yellow Go Wellington buses.

ROGER BLAKELEY/SUPPLIED The diesels will be operate until Wellington's new Wrightspeed hybrid buses are introduced, hopefully by July next year (FILE PHOTO).

The council confirmed six surplus diesel buses had been brought south from Auckland to meet increasing demand on some routes, including Karori, Wilton and some school services.

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"Two [of the buses] are five-years-old, and the other four are 20-years-old," a council spokesman said in a statement.

DAVID UNWIN/STUFF Tranzit, which will take over many NZ Bus contracts from July, will operate Euro 6 diesels, which meet the highest global emission standards (FILE PHOTO).

"The extra demand has built in small increments, and in response we have added individual services. This has been going on for some time."

The first of the six buses arrived in Wellington last October, the spokesman said.

But from July next year, some of them will not be allowed to operate on Wellington's streets because they will not be considered environmentally-friendly enough.

SUPPLIED Greater Wellington Regional Council sustainable transport committee chairowman Barbara Donaldson says the diesel bus replacements are "not desirable".

New rules are set to kick in that will limit all diesel buses in Wellington to at least a Euro 4 emission standard, for existing bus operators, and Euro 5 or 6 - the highest global emission standard - for new operators.

The six buses brought into Wellington recently have emissions standards ranging between Euro 2 and 5, the council said. The Euro 3 standard was introduced 17 years ago.

Councillor Barbara Donaldson, chairwoman of its sustainable transport committee, acknowledged the recent additions to the bus fleet were "not desirable".

SUPPLIED Sustainable transport committee deputy chairman Daran Ponter says the situation is frustrating.

"At the moment we are really trying to meet the capacity demands the best we can, but we agree that having buses that old is not where we want to go."

Committee deputy chairman Daran Ponter said he was not happy the buses were operating in the meantime even though current rules allowed it.

The situation was frustrating, he said.

"People are rightfully concerned and annoyed. They're seeing these spilly diesels arriving and wondering what's going on."

NZ Bus chief executive Zane Fulljames confirmed the council approached his company to source extra buses.

"We have regular conversations with the local government authorities around capacity. If there's additional capacity required, we work with them to manage that."

Surplus Euro 6 diesels will also be brought into Wellington from Auckland to replace the city's electric trolley buses, which will be retired at the end of the month.

The 60 trolleys are to be be fitted with hybrid Wrightspeed motors by owners NZ Bus, and it is hoped they will be operating by July next year.