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Police are appealing to the public for information after a suspicious fire destroyed several Wellington buses and caused millions of dollars in damage this morning.

Acting detective sergeant Caroline Martin said police believe the early morning fire began when someone gained entry to one of the parked buses at the Karori bus depot and set it on fire.

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The blaze was reported to the Fire Service about 3am and was extinguished relatively quickly - preventing damage to surrounding properties - but at least four Go Wellington buses in the depot were destroyed.

NZ Bus estimated the damage to cost between $2 and $2.5 million, with all five buses involved being relatively new additions to their Go Wellington fleet.

Ms Martin said a scene examination of the fire was completed this morning.

"We're following up a number of lines of enquiry in relation to this matter."

She said it was too early to determine whether an accelerant had been used.

"Police are appealing for anyone who may have seen anything unusual in the Darwin St area, or have any other information in relation to this matter to come forward and speak to Wellington police."

Residents in the area surrounding the bus depot, which is fenced off at night, were concerned about the incident and were interested in finding out what had happened, she said.

"There is a close proximity between the residential addresses and the buses, and it potentially could have had a catastrophic [outcome]."

NZ Bus chief operating officer Rachel Drew said at least four buses had been "completely written off".

"Three of them were brand new, only 3- to 6-months-old. The other ones are only a couple of years old."

A fifth bus, which was parked behind the bus police believe was set on fire, sustained damage around its front and could probably be salvaged, Ms Drew said.

"There would be $2 to $2.5 million of damage."

While the fire and resulting damage had been "hugely disappointing and frustrating", Ms Drew said she was relieved there had been no injuries or damage to surrounding houses.

A security review at all Wellington bus depots had been launched as a result of the fire, she said.

"We're bringing in security tonight to make sure that the scene's reserved."

There had been no previous problems with security at the Darwin St site, she said.

- APNZ