Tributes flow for 'warrior' Jim Molan

Decorated soldier and Liberal senator Jim Molan has been praised for a life of service to Australia following his death aged 72.

Senator Molan died on Monday surrounded by family, following a "sudden and rapid" decline in his health after Christmas.

Prominent in his later years spearheading Operation Sovereign Borders before joining the Senate, perhaps his greatest honour came in the Iraq invasion where he served as chief of operations for the coalition forces.

Senator Molan, who represented NSW, was a major-general in the army, serving across a 40-year period in Papua New Guinea, Indonesia, East Timor, Malaysia, Germany and the United States along with Iraq.

Former RSL president and army veteran James Brown said Senator Molan was a "general's general" and a "blunt instrument".

"He was someone you sent to get the mission done ... there was a view he was the general you wanted in the glass cabinet to break in time of war," he told Sky News.

"He was focused absolutely on Australian security, on the security of our allies, on doing what he could do in the positions he held to make sure that Australians could live safely."

He was awarded a Distinguished Service Cross by Australia and the Legion of Merit by the US, along with an Order of Australia.

Former prime minister Tony Abbott said Senator Molan was critical to the crackdown on asylum seekers arriving by boat.

"Jim having been involved in so many complicated military operations could see immediately this was one of the reasons that had bedevilled the former Labor government's approach to border protection and was determined we wouldn't make the same mistake," he told Nine Radio.

Another former prime minister, John Howard, also expressed his sorrow for the "energetic, passionate Australian (who) gave so much to his country".

He described the senator as a "plain-speaking man" and "a true local citizen" following his years of service, including as a volunteer firefighter and rescue helicopter pilot.

Senator Molan first joined the Senate in 2017, and was re-elected at the 2022 federal election.

Opposition Leader Peter Dutton described his colleague as a "soldier's soldier".

"Whether it was on the battlefield, in the political arena or on the media stage, Jim was admired for his discernment, leadership and unfailingly courteous manner," he said.

He was diagnosed with cancer in 2021, taking leave from the Senate for treatment.

His family thanked people for their thoughts.

"Our loss is immeasurable, but we are comforted in our memories of a full life courageously lived, devoted to family and in service of the country he loved," they said in a statement.

Prime Minister Anthony Albanese sent condolences to his family and friends.

"Jim Molan lived his life in service of our country ... he was a man of principle and a politician of conviction," he tweeted.

Senator Molan was also a champion of education and research into stillbirth.

His replacement in the Senate is likely to be held up by the NSW state election in March, as it requires the selection of a nominee by the Liberal party and the endorsement of a joint sitting of the state parliament.

Liberal senator Jane Hume said while it would be terrific for a qualified, high calibre woman to follow in the late senator's footsteps, it was too early and "possibly inappropriate" to be talking about his replacement.

"Today, our hearts and our minds and our thoughts are with Jim's family," she said.