Sydney, Australia's biggest metropolis, is a place of sophisticated infrastructure - so it often surprises visitors that parts of the city can't be reached by road.
Admittedly, very few such isolated pockets survive. And, to be fair to city planners, people who live in such hideaways usually do so specifically because they crave isolation.
But, for the rest of us, these tranquil time warps seem made for easy but close-to-urban-bustle exploration. Fortunately, those who live there aren't hermits and welcome well-behaved visitors.
Take Dangar Island, for instance. This Hawkesbury River pinprick seems a world removed from the Sydney upon which (depending on flight paths used) some visitors arriving by air feast their eyes.
While some tourists gaze down on the Harbour Bridge, Opera House and the impressive skyline of one of the southern hemisphere's greatest cities, this vista is a world away from Dangar Island and the mail run of the last of Sydney's riverboat postmen.
And that's the way Dangar Island's 200 residents like things.
They announce proudly that theirs is the only Sydney suburb without cars (at last count there were three golf buggies, a ute used for garbage collection and a fire truck). However, most residents have boats.
The 30.8-hectare island boasts a licensed lawn bowls club, a well-regarded cafe-restaurant, a safe swimming beach, an offshore kayaking route, bush walks along trails bordered by native fauna, holiday homes for rent and a ferry service (taking five minutes to Brooklyn) that allows residents to work in far bigger communities.
There's a water taxi for commuters who miss the ferry. Trains from Brooklyn's Hawkesbury River station take about an hour to reach Sydney.
The same company that runs Dangar Island's ferry service is contracted by Australia Post to take water-borne mail, five days a week, to recipients who can't be reached by road.
Passengers can go along for the ride, accompanying the self-described "Last Riverboat Postman" as parcels and letters are delivered and collected.
"Some of our passengers are Sydneysiders, some are on holiday from interstate and some are overseas tourists," says Jim Ray, as he skippers the MV Hawkesbury on the mail run. [The vessel is filling in for the same company's MV Hawkesbury Explorer which usually does the run but is presently undergoing maintenance.]
"We often encounter Sydney people who bring visitors from elsewhere for an unusual experience," adds Ray.
"People who don't know Sydney more often go on harbour ferries or take a conventional sightseeing cruise from the city. Many don't even know we exist in Sydney's backyard."
According to Ray, "people get to see a tremendously scenic part of Sydney that they'd otherwise miss - and, of course, it's a relatively cheap day".
Ray is one of several skippers who become the "last riverboat postman" when on duty.
He's helped by one crewman who serves morning tea and biscuits to passengers as well as handing mail to residents who meet the mail boat at the end of their jetties.
Tourists often walk from the marina, where the mail run departs, through the adjoining small town of Brooklyn. Several local eateries serve oysters, for which the Hawkesbury River is famous, and showcase other seafood.
Brooklyn - with a railway station, relatively light road traffic, art galleries, restaurants, a pub, a motel and a well-stocked second-hand bookshop - has its roots in oyster-farming.
Passengers sit in ferry-style surrounds while mail is piled high, some of it in canvas bags, on several rows of seats.
A few minutes after leaving Brooklyn, the ferry passes under a Sydney landmark, the 123 year-old Hawkesbury River Rail Bridge which was one of the world's longest iron bridges when built and remains the longest purpose-built rail bridge in the NSW network.
I've previously travelled on the mail run in gloriously sunny weather, with the glistening and almost flat surface of the water resembling hammered metal. But, this time, relentlessly driving rain and mist are my companions for the entire voyage.
Happily, my fellow passengers' enjoyment of their oddball adventure isn't dampened.
And, as Ray notes with a grin, "whatever the weather, the mail has to get through".
"It's not up to us to decide whether or not people should get their letters."
Isolated dwellings ease by. Tumbledown fishing shacks and opulent-looking mansions co-exist easily in these parts.
Some jetties jut from solitary homes surrounded by thick bushland. Others serve small communities such as Milsons Passage with several dozen houses - a combination of permanent residences, weekenders and holiday homes.
Our vessel glides past native bush, strangely-shaped rocky outcrops, lone houses and waterproofed fishermen in their tinnies.
"We don't stop at every jetty," explains crewman Jason Freeman.
"If we've nothing to deliver we pass 'em by - unless they're standing on their jetties and waving to indicate they have something for us to pick up."
Sometimes there are about 10 stops, sometimes twice as many.
Residents are often accompanied by their dogs when they come to collect their mail - and Jason routinely rewards pets with a biscuit.
Besides mail, the vessel delivers groceries, machine parts and other supplies. But communities close to the motorway are skipped altogether.
"They're accessible by road so receive normal deliveries," explains Freeman.
After a mail drop at a place called Marlow Creek, a Gosford suburb about 60km to Sydney's north, we turn around and head back to Brooklyn.
"It's all so gorgeously quaint," sighs a woman from Canada sitting next to me.
"It's a part of Sydney I'd never have seen if I hadn't asked a local friend for advice."
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