Mr Fischer died at the Albury-Wodonga Cancer Centre on Thursday, surrounded by close family members.

The 73-year-old had been fighting acute leukemia for the past 10 months, and cancer generally for the past decade.

Prime Minister Scott Morrison described the former Nationals leader as a dear friend who would be sorely missed.

"Tim Fischer was a big Australian in every sense of the word," Mr Morrison said.

"Big in stature, big in his belief, big in his passion, big in his vision for what Australians could achieve, and big in his view of Australia's place in the world.

The prime minister has offered Mr Fischer's family - wife Judy and sons Harrison and Dominic - a state funeral.

He described him as an "all-in conviction politician."

"Thank you Tim, we loved you very much," Mr Morrison said.

After returning from a tour of duty in Vietnam, the former soldier entered the NSW state parliament at 24 before being elected to federal parliament in 1984, leading the federal Nationals from 1990 to 1999.

He was deputy prime minister in John Howard's government from 1996 to 1999.

Mr Howard described Mr Fischer as an "authentic Australian in so many different ways".

"He was able to identify the broader national picture, even on issues that mightn't immediately be appealing to his own constituency," he told 2GB Radio.

Mr Fischer notably supported Mr Howard in staring down his own angry rural constituents during the introduction of Australia's tough gun laws following the Port Arthur massacre in 1996.

"There are many Australians who are alive today because of legislation change, because of that tough decision that Tim Fischer took, that courage that he showed," Nationals leader Michael McCormack said.

Mr McCormack said Australia had lost one of its finest.

"Tim embodied loyalty, kindness and courage. Regional Australia had no better friend," Mr McCormack said.

Noting Mr Fischer's love of trains, he added: "While Tim has left the station today, his legacy will live on."

Former prime minister Kevin Rudd was responsible for appointing Mr Fischer as Australia's "first and best" ambassador to the Holy See in 2008.

"I will never forget him in his beloved Akubra with me in audience with the Pope. A good Australian," Mr Rudd said.

His service was also recognised by Pope Benedict who made him a papal knight in 2012.

"Tim never left you in any doubt what he believed in ... you always knew where Tim was coming from," Mr Howard said.

When he retired from politics, Mr Fischer worked tirelessly for many causes, including autism, veterans affairs and agricultural research.

He was lauded on Thursday by many of those groups he supported, including farmers, rural doctors and Autism Spectrum Australia.

National Farmers Federation president Fiona Simson said Mr Fischer should be recognised first and foremost for his service as a conscript and platoon commander in the Vietnam war.

"There are few other people who as accurately typify what it means to be an Australian as Tim Fischer," she said.

NSW Farmers described him as "A true gentleman and a giant in representing rural and regional Australia."

Business and transport industry figures were also full of praise.

"Tim Fischer's long-term vision for the country was inspiring. He led with conviction, courage and integrity," Business Council chief Jennifer Westacott said.

Australian Associated Press