SYDNEY • On a quiet street in Australia's richest suburb, a narrow lane leads off the road and through a thick forest towards the glittering foreshore of Sydney harbour.

The lane ends at the waterside, leaving pedestrians facing three of the country's most unusual houses. Here, bobbing side-by-side on the gently lapping waters, are three of the last remaining houseboats in Sydney.

These unusual residences - they do not travel on sea and are permanently attached to the shoreline - are in the exclusive suburb of Mosman in north Sydney.

The combined income of the suburb's residents is A$2.7 billion (S$2.63 billion) - the highest in the country. Homes here cost an average of A$2.6 million, with some of them going for as much as A$20 million.

In this property-obsessed city, a harbour view is the biggest prize of all, and can add millions of dollars to a home's value.

But these three sets of floating residents have found a more affordable way to gain a trophy view.

From the wooden deck of her houseboat, Ms Maureen Young has unparalleled views of the harbour and the surrounding leafy foreshore. And she has no risk of a new mansion arising from nowhere to block her waterfront view.

"It's unique," she said. "The views are fantastic. It is relaxing. As a way of life, people just love it."

But her floating residence - which is entered via a wooden gangway - is part of a disappearing type of abode in Sydney.

HISTORY'S INCONVENIENT LEGACY

There are now just four remaining residential houseboats, scattered about the harbour.

They are believed to date back to around the 1910s, and point to a time when local residents were far less affluent.

The harbour was once lined with clusters of houseboats, many of which were first inhabited by people trying to escape homelessness during the Great Depression of 1929-1939. The state authority has attempted to phase them out.

The three Mosman houseboats sit in a small inlet called Pearl Bay, where there were about 12 in the 1950s but only three by the 1980s. Many people in the city appreciate them as curious structures that add to the neighbourhood's character. But, as a book on the history of the area once observed, some residents and councillors have long seen them as an eyesore, and have tried to have their licences cancelled.

"The council would have scuttled them all if it could, but to the relief of those who liked them, the New South Wales Maritime Services Board declined to relinquish its authority over houseboats, and continued to renew existing licences," according to a book on the history of Mosman by Australian historian Gavin Souter.

The New South Wales state government's reason for wanting to phase out residential houseboats is that it is concerned about the environmental impact on the waterways, particularly if large numbers of people were to swap their home on land for one on the water.

The state allows people to spend 21 days at a time on holiday houseboats, something that is popular with tourists, and as weekend getaways for locals. The only exceptions are the last four residential houseboats in the harbour.

According to a spokesman, the rule was designed "to ensure NSW waterways remain clean and comfortable for the entire boating community".

The spokesman said: "There are four houseboats permanently moored in Sydney's Middle Harbour which are exempt from these rules due to historical lease agreements."

These vessels are permanently connected to shore utilities such as power and sewage, and have access to local council waste removal services.

Aside from the three houseboats in Mosman, the fourth is nearby, at the foot of cliffs in the suburb of Clontarf.

But there is no guarantee that all of the remaining households will continue to have their leases renewed by the state authorities.

In 2011, Mosman council considered a plan to phase out the three houseboats. But the council received 10 public submissions, all of which rejected the proposal.

"The submissions argue that the houseboats are part of the rich cultural heritage of Pearl Bay, and that they should be retained," said a council report.

The state's maritime authority told the council in 2012 that it had issued a new 20-year lease for one of the houseboats, and was renewing the other two.

"It would be anticipated that the houseboats would have the ability to remain on site for a minimum of 20 years," the government told the council.

Ms Young's floating house, named Tanderra, has been particularly controversial.

It is in a rundown state, with rotting wooden walls and a tarpaulin covering the roof. A local councillor once said that it should be "towed out to sea and sunk".

Ms Young, a retiree who bought the houseboat for about A$123,000 in 1983, rented it out initially, and then moved there permanently in 1988. She said she wanted to renovate but has been locked in a long-running legal battle with the state government over the details of her leasehold.

She would not discuss her legal battle, but would only say: "It is so derelict that it is hard to live here. The architect's plans for the boat are magnificent."

CHEAP DIGS FOR RETIREES

In the rest of the country, houseboats have proven tempting as a cheap escape from Australia's soaring property prices in recent years, particularly for retirees, with some costing as little as A$50,000.

Most houseboats have generators and are equipped with working kitchens and bathrooms but there are typically strict requirements about disposing of dirty water without damaging the environment.

Australia has a large river system, with numerous wide, gentle waterways that have been relatively unaffected by development. These have proven popular spots for holiday houseboats, mainly for renting to tourists.

Some of these houseboats can cost A$2 million or more, and include luxury suites and outdoor spas.

But like New South Wales, some of the other states discourage people from living in houseboats on rivers.

The state of Western Australia, for instance, warned in 2001 that permanent houseboat residence on waterways "may create an undesirable precedent".

"It may result in an increase in houseboat numbers, especially in popular areas, leading to possible alienation of significant areas of public waterways," it said in a policy document.

"Houseboat activities may also damage fragile waterbird habitats, obstruct public access along foreshores, and impact on the social values of waterways, such as fishing and recreation," it added.

The state of South Australia, however, has taken a different approach and has been more permissive towards people living on houseboats.

Numerous people float up and down the Murray River - the world's third-longest river - or live aboard boats at permanent moorings. The state charges annual registration fees ranging from A$115 for a 5m boat to A$455 for a 15m boat.

Some residents in South Australia, such as Mr Michael Heuzenroeder and his wife Sue Holland, live on land but use their houseboat, which is worth between A$50,000 and A$60,000, for recreation.

Mr Heuzenroeder, 60, a retired public servant, said they had taken their boat on lengthy voyages up the Murray River. But often, he said, they simply spend a weekend staying on the boat at its mooring.

"I like being on the water," he said. "It's a very gentle ride on the river. I enjoy the nature and the creatures you see on the bank - the kangaroos, emus, birds, snakes, goannas." Goannas are large lizards.

Mr Heuzenroeder said "a lot of people" in the area - particularly retirees - live on houseboats.

"It's very comfortable," he said. "You have everything you'd have in a normal house."