Three-wheeled vehicles designed to drive on the footpath are part of a New Zealand Post initiative being rolled out across the country to keep up with its "growing" parcel business.

New Plymouth will be the first city to test the new eco-friendly vehicles, which were designed to deliver both parcels and mail simultaneously to residential addresses and businesses.

As online shopping orders continued to boost parcel numbers, the pilot run in New Plymouth would be a test for the new combined system, New Zealand Post said.

Ashley Smout, delivery officer for New Zealand Post, said the new system should help "avoid the disappointment of a missed delivery".

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"Customers are buying goods online in numbers we've never seen before and our parcels business is growing.

"We want to give customers more choice about where and when they get their parcels."

From the start of June, new delivery vehicles would be out on the street in New Plymouth, and by early July, delivery of letters and parcels to residential addresses would be combined.

"The new technology we're developing will help avoid the disappointment of a missed delivery, as customers will be able to tell us in advance what they want us to do with their parcels," said Ms Smout.

"Previous trials have shown the eco-vehicles are safe to use on the footpath. Our drivers receive safety training, and they will give way to all other footpath users."

Staff driving the eco-vehicles will have a specially designed uniform consisting of a black long-sleeved padded polo shirt and padded track pants with a combined New Zealand Post and CourierPost emblem.