For a travel destination famous for offering the adrenaline rush of extreme sports, from bungee jumping to the parachute drop, it’s an unlikely tourist activity – but an irresistible one. If you’re travelling in New Zealand, don’t miss out on the chance to deliver the mail. By boat.

It happens in the Queen Charlotte Sound, part of the Marlborough Sounds in the stretch of water that separates New Zealand’s North and South Islands. For over 160 years, New Zealand Post has ensured the handful of families who live on the bays and inlets of the sound receive the same mail service as every other resident of the country, no matter that they live in isolated homes accessible only by boat. Six days a week, the mailboat leaves from Picton, the skipper doubling as postman for the three- or four-hour voyage – and these days passengers can come along for the ride.

The truth is, it’s the passengers who make it possible. If it weren’t for them, the mailboat run would have been abandoned decades ago, deemed hopelessly uneconomic. Now the boat company, Beachcomber Cruises, effectively subsidises the service and keeps it afloat. They’re glad you’re there, so you’re invited to come up front, stand next to the skipper, ask anything you want – and help out.

Even on a drizzly day, the landscape is spectacular, the tree-covered hillsides sloping down to the water’s edge as the boat weaves its way to destinations with names that evoke smugglers’ tales of a bygone age: Curious Cove, Lazy Fish and Maraetai Bay. From a distance, you can see the specific house, perched above the shoreline, that the captain is aiming for as he approaches to make a delivery. Each one has a jetty with its own number, the Queen Charlotte equivalent of a postcode.

As the boat gets nearer, someone emerges to greet it. I ask the skipper, Iain– a British-born firefighter in Wellington who saves up his entire annual holiday allocation so he can head southwards and work the Picton mailboat in summer – if there’s a timetable, so that residents know exactly when to come out to pick up their post. No, he says. They have a mechanism that’s far more reliable.

On delivery days, the families’ dogs wait expectantly, heading to the jetty when they get so much as a sniff of the approaching vessel. It’s not only the approach of a visitor to these isolated households that gets the animals excited. The dogs know that Iain and his fellow skippers come bearing gifts.

At our first stop, I watch as the boat pulls alongside the jetty, Iain opens a side window and asks after the health of the woman who’s standing ready. She hands him a mailbag with her outgoing post as he hands her an identical bag with her incoming mail. The swap complete, he then transacts the main business of the day – handing her pet a chunky, triangular dog biscuit. It’s clearly a highlight of the animal’s week.

And on it goes, jetty by jetty, the swapping of the mailbags, sometimes accompanied by a few parcels, including Amazon deliveries. When we get to the house on Arapaoa Island, Iain lets me hand over several boxes through the boat’s open side door, including one bulk order of toilet paper. If it wasn’t for Iain and the mailboat, the family on Arapaoa would have been caught short.

The houses are so dispersed that only a third of the registered addresses get a delivery on any given day, so each gets two deliveries a week. It takes that long to reach them. And whatever course the skipper plots, it’s the scenic route. We pass a series of floating ropes on the surface of the water – a mussel farm – a derelict whaling station and an island that’s home to fur seals, several of whom are in residence as we pass. Dolphins and orcas have been known to swim alongside the mailboat, coming along for the ride.

For the return to Picton, the boat turns into a bus, picking up hikers who have been walking the stunning Queen Charlotte Track, giving them a lift after they’ve had a drink at Furneaux Lodge or a swim at Punga Cove. Take the mailboat and you won’t need your heart rate checked, as you might after undertaking the daredevil activities that draw so many to New Zealand – but you will be charmed.

• Tickets to the Mail Boat Cruise by Beachcomber Cruises start from NZ$132. The cruise departs from Picton in New Zealand’s South Island at 1.30pm, and operates Monday to Saturday during the summer season; and Monday, Wednesday and Friday during the winter season