The former army general and architect of the Coalition's military-led border protection policies lost his almost two-year cancer fight earlier this week, after a "sudden and rapid" decline in his health after Christmas.

His passing creates a vacancy in the Senate, which will be filled by a Liberal from NSW.

News of his death prompted an outpouring of tributes from across the political divide.

Living in Royalla, just south of the ACT, Senator Molan had a close association with Liberals in Canberra.

ACT opposition leader Elizabeth Lee said despite his illness, Senator Molan ensured the Canberra Liberals had an advocate in the Federal Parliament after Zed Seselja's defeat at last year's election left them without a seat.

"It will leave a huge hole and it is a deep, deep loss," Ms Lee said, adding she was confident Liberal leader Peter Dutton, deputy leader Sussan Ley and Hume MP Angus Taylor would speak up for Canberra.

Mr Dutton led tributes to his late colleague, saying Australia had "lost a patriot, a decent and honourable man and above all and most importantly an incredible family man".

Prime Minister Anthony Albanese said Senator Molan had "lived his life in service to the country".

In a statement announcing the news on Tuesday morning, Senator Molan's family said their loss was "immeasurable".

"He was many things - a soldier, a pilot, an author, a volunteer firefighter, and a senator. Most of all, he was an adored husband, father, grandfather and brother," the statement read.

In a sign of the respect he commanded from his political opponents, Labor's past and present MPs for Eden-Monaro, Mike Kelly and Kristy McBain, paid warm tributes to Senator Molan.

"Our politics didn't align but our love of SE (south-east) NSW and our desire to represent the views of our communities was something we had in common," Ms McBain said.

The father-of-four served in the Australian Army for 40 years, which included deployments to Papua New Guinea, Indonesia, East Timor, Malaysia, Germany, United States and Iraq.

In 2004, he served for a year as the chief of operations of coalition forces in Iraq.

In a statement, Department of Defence secretary Greg Moriarty and Chief of the Defence Force Angus Campbell said Senator Molan left behind a "substantial legacy and will be remembered for his strong leadership, commitment to service, and as a man who served his country with honour".

Senator Molan's involvement in politics started in 2013 when he was appointed by the Abbott government to design Operation Sovereign Borders, the military-led policy to stop asylum seeker boats reaching Australia.

He entered Federal Parliament in 2018, replacing Nationals senator Fiona Nash after she was caught up in the section 44 citizenship saga.

After being relegated to an unwinnable fourth spot on the NSW Liberal ticket ahead of the 2019 election, he broke ranks and encouraged people to vote for him directly.

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A huge first preference vote wasn't enough to get him across the line, but he would eventually return to the upper house after the resignation of Liberal senator Arthur Sinodinos.

He used his position in parliament to speak up about defence and national security issues, in particular the rise of China. A political conservative, he supported same-sex marriage and Australia becoming a republic.

A NSW Liberal Party spokesman said it would run a process to select a candidate to fill the casual vacancy in "due course".

Senator Molan is survived by his wife, Anne, and four children, Sarah, Erin, Felicity and Michael.

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