McCullough worked as a neuroscientist in the United States before turning to writing full-time. The Thorn Birds, a romantic Australian saga published in 1977, has sold 30 million copies worldwide and is the highest-selling Australian book, helped by the popular 1983 mini-series.

The paperback rights for The Thorn Birds, McCullough's second novel, were sold at auction for a then record $US1.9 million.

Her 25 books included a deeply researched fictional series set in Ancient Rome, which won her the admiration of readers including former NSW Premier Bob Carr and Newt Gingrich, the former Republican speaker of the US House of Representatives, and was credited with renewing interest in ancient history among university students.

McCullough lived on Norfolk Island for most of the past 40 years and married Norfolk Islander Ric Robinson in 1983.

She died on Thursday afternoon after suffering a series of small strokes. She had lost her sight from macular degeneration and was restricted to a wheelchair.  

In 2013 she published her final book, the novel Bittersweet, about four sisters in 1920s New South Wales.

Her publisher at HarperCollins Australia, Shona Martyn, said McCullough had been dictating a sequel, set around World War II, into an old-fashioned dictaphone but had only completed a third of the novel when she died.

Sydney literary agent Selwa Anthony, one of McCullough's closest friends, said she had encouraged McCullough to return to writing the mainstream women's fiction that had made her name.

"She finally gave in and Bittersweet took her back to Colleen's storytelling. She said she would call it 'chook lit' and I said 'Don't you dare.'

"The book has done extremely well [with Australian sales at 65,000 copies] and pulled in a lot of mainstream readers who had moved away."

Anthony was working as a buyer at Graham's bookshop in Sydney when The Thorn Birds was published.

"I was so excited when I read it before it was published that I said I'll take 500 copies and the sales rep nearly fell over. We had already sold half of them when she came in and signed books for two hours."

Australian readers loved The Thorn Birds because McCullough wrote about Australia in a way that hadn't been done.

"I thought The Thorn Birds was as good as Gone with the Wind but set in our landscape," Anthony said. "It was over the top and a real page-turner.

"She encouraged me to become a literary agent and gave me the courage to say I'm going to discover lots of popular Australian fiction writers."

McCullough turned to the Roman novels she had always wanted to write when she had enough money to live and employ researchers to help her gather the vast amount of historical information she used. She accumulated an impressive private library of material on the Roman republic.

As NSW premier, Bob Carr quoted from McCullough's biography of Sir Roden Cutler at the funeral of the former NSW governor. This led to a friendship based on a shared passion for Roman history.

Carr read all but one of McCullough's seven Roman books ("that one had one siege too many").

"Her research was unimpeachable," he said. "She spent 13 years before she wrote The First Man in Rome in the most systematised research.

"She took rolls of telex paper, got all the resources of the Roman Republic and typed up everything that happened in every year. I think her background as a medical researcher explained that cast of mind."

"The artistry became more finely honed as she got to the end of the series and her touch became lighter and livelier. She outshone others who have nibbled at stories of the Roman Republic. [Scottish author] Allan Massie was pallid by contrast." 

Carr said McCullough was "easy to get to know; she exuberantly shared her life story and gave a colourful account of her father who was revealed to have had two families. She had a great partnership with Ric, which was hugely apparent, and they were great company." 

While publishers continued wanting her to write another Thorn Birds, McCullough went on to write a series of detective thrillers set in 1960s America.

"She wrote what she wanted," Anthony said. 

Anthony described her friend as "a big lady who was open to interviews and the passion was always there. She had a big life and leaves a big legacy through her wonderful books and her generous spirit; she was always helping people."

The Prime Minister, Tony Abbott, paid public tribute, saying, "Colleen McCullough AO was one of Australia's most-read authors. She enthralled readers for decades.

"Colleen McCullough was a unique Australian personality and Norfolk Island's most famous resident.

"Her book Roden Cutler VC will always be required reading for those who want to know the story of one of Australia's great heroes and NSW's longest-serving governor.

"She will be missed."

Susan Wyndham is the Sydney Morning Herald's Literary Editor

smh.com.au