Len Evans, who as a writer, vineyard owner, restaurateur and promoter was for almost half a century Australia's wine ambassador to the world, died Thursday in Newcastle, in New South Wales, Australia. He was 75 and lived in Pokolbin, in the Hunter Valley, once considered Australia's best wine region.

The cause was a heart attack he had while driving to pick up a relative who had had surgery at a Newcastle hospital, according to his son-in-law Steve Waterson.

Mr. Evans began his wine career in 1960 washing glasses in a Sydney pub. He soon moved to the Chevron Hilton Hotel in Sydney, where as assistant beverage manager he put together Australia's first important restaurant wine cellars.

He founded the Australian Wine Bureau in 1965, and wine sales doubled in Australia in five years. In 1962, he began the country's first wine column, ''The Cellarmaster,'' in The Sydney Bulletin. He published the first major encyclopedia of Australian wine in 1973.

He wrote hundreds of articles on wine for publications around the world as well as a series of books on wine and food, most of them larded with anecdotes from his own adventures in the wine world. His ''Complete Book of Australian Wines'' went through seven editions.

In 1969, with other investors, he started the Rothbury Estate, a winery and vineyard in the Hunter Valley, about 200 miles north of Sydney. That same year he opened Bulletin Place, a restaurant and retail wine shop in Sydney, and for a time the shop was thought of as Australia's unofficial wine capital.

In the 1970's, backed by a wealthy friend, Peter Fox, Mr. Evans founded the Evans Wine Company. The company acquired Rothbury as well as two small chateaux in Bordeaux and a vineyard in the Napa Valley. It also invested in Petaluma, a famous winery near Adelaide in South Australia. There was an agreement, too, to acquire a famous Bordeaux property, Chateau Lascombes, from the New York owners.

Then, in December 1981, Mr. Fox was killed in a car accident. It turned out that he had undisclosed financial problems, and shortly, the Evans Wine Company's growing empire was gone.

Mr. Evans was able to save Rothbury Estate and Petaluma. His company, now named Evans Family Winery, survived some perilous years and eventually prospered. In 1982 he was awarded the Order of the British Empire for his service to the wine industry.

Leonard Paul Evans was born in Felixstowe, England, to Welsh parents. His father served in the Royal Flying Corps in World War I and later, as a civilian, held administrative posts in the Royal Air Force. The younger Mr. Evans served as a physical fitness instructor in the R.A.F., hoping one day to become a professional golfer. In 1953, despairing of a golfing career, he moved to New Zealand, where he cut timber and culled deer herds. Moving to Australia in 1955, he worked in the forests for a time, built farm fences, made car mufflers, wrote radio and television scripts and gave golf lessons, all before devoting himself to wine.

As a youth, Mr. Evans had turned down a scholarship to Cambridge to play golf. Had he gone to the university, he said, he would have studied architecture. Instead, as an amateur, he designed wineries and his own home in Pokolbin, built entirely of immense timbers from a harbor pier dismantled to make way for the Sydney opera house. He called his home Loggerheads.

His survivors include his wife, Patricia, known as Trish; two daughters and a son.

Despite long-term heart problems, Mr. Evans insisted on living his life to the fullest. ''He had no intention of wasting his time on a bad bottle,'' said Christine Maher, a friend and business associate.

In print and in person, he was a master raconteur and an irrepressible bon vivant. Some of his parties were said to last two days. When visiting friends he would reject the wines they had carefully chosen for dinner and demand that they produce the very best they owned. The next day his hosts would receive a contrite note in a package of even better wines from his own vast cellar.

Photo: Len Evans founded the Australian Wine Bureau in 1965.