Sydney - It's lunchtime in Sydney and there are more fitness-conscious office workers in the pool doing laps than there are in the park doing circuits in their running shoes.

The passion for following the black line is reflected in the Olympic medal tally. Australian swimmers have won 40 per cent of all the Olympic medals awarded at all the games (141 out of 350).

Those not quite good enough to get into the national team but whose enthusiasm for the water is undiminished, often go on to excel in open water swimming, where it's not speed but endurance that counts.

This year Australians took the first four places in the Manhattan Island Marathon Swim in New York, the longest open-water race in the world and the most prestigious.

John Van Wisse, who also won the 45-kilometre race around Manhattan Island in 2000, spent 7 hours 30 minutes in the water this year.

It's a discipline of mind-over-matter triumphs where victory goes to the competitor best able to block out the cold, the boredom, the fatigue and the loneliness.

"You try not to think too much," 35-year-old Van Wisse said of his July triumph. "Otherwise, you know how far you have to go and just how cold the water is, so I just zone out, let time go by and get my rhythm."

Psychologists would probably say that continent-sized Australia, a sparsely populated country with big distances between habitations, is a good place to breed long-distance athletes.

But the ineluctable truth is that success in the open-water is determined by zillions of training laps in the Olympic-size pools that dot Australia 's cities.

Men aren't particularly advantaged in open-water swimming. And people remain competitive into their 30s and 40s. Tammy Van Wisse, John's sister, retired at 38 after winning the Manhattan Island swim in 1997.

Tammy is the first person to swim Bass Strait, which separates mainland Australia and Tasmania. She also holds records for swimming Scotland's Loch Ness, New Zealand's Cook Strait and the English Channel.

It's a pity for Tammy, but her achievements were always in the shadow of those of fellow Australian Susie Maroney, who a year after her retirement spent 8 hours in a Sydney pool to rack up an astonishing 675 laps.

Maroney is the Lance Armstrong of ocean swimming, holding the record for the longest distance swum in 24 hours (93 kilometres) and the fastest double crossing of the English Channel by a woman. She has swum from Jamaica to Cuba, and from Cuba to the United States.

She also holds the world record - 38 hours 27 minutes - for the longest non-stop open-water swim.

Where brother John zones out, Tammy stays focused with music. "That gets me through the low periods," she said. "I just click on a song in my head and swim to the beat."

Maroney, who needed a bigger song library than anyone, also replayed whole feature films in her imagination. In her Mexico-Cuba swim, Maroney visualized every scene in the movie Titanic.

Morty Berger, the organizer of the Manhattan Island swim, takes his cap off to Australians, who made up no less than a quarter of the field in this year's classic.

He said Australia set the "gold standard" of marathon swimming but had no suggestions as ton why this should be so.

The climate certainly is a factor. It's warm enough for outdoor pools to be open all year-round. There's also a penchant for setting world records - no matter for what.

Australia holds the world skipping champion and the world long-distance motorcycle jumping champion.

Perhaps the intensity of competition in national teams is part of the answer. Maroney failed to score a place in the swimming team but more than made up for that setback by beating all-comers in the open water. (dpa)