New Zealand Post is to extend a trial of customised mobility scooters that allow posties to deliver both parcels and letters, helping the state-owned mail service cut costs as the internet shrinks letter volumes.

The two and three-wheeled electric scooters were trialled in Wellington's Hutt Valley last year. NZ Post said today it will hold more extensive trials of combining letter and parcel deliveries, including a pilot in New Plytmouth to test the vehicles further.

Domestic letter volumes shrank 7 percent to 642 million pieces last year and are forecast to drop below an annual 500 million in the next three years as consumers switch to the internet, email and social media for everything from paying bills to sending birthday greetings and keeping in touch with loved ones.

NZ Post's 2014 profit declined 12 percent in 2014 and would have dropped further without the contribution of earnings from its Kiwibank unit.

The postal service today confirmed plans to reduce standard mail deliveries to every second day for households in urban areas, starting in July an d rolling out to smaller centres over the next two years. Six-day-a-week deliveries would continue for priority mail and courier parcels.

"We're making these changes in response to people sending fewer letters," said NZ Post mail and communications chief operating officer Ashley Smout. "Our analysis shows the average household receives letters just three days a week."