Retired Army major general Jim Molan is set to return to Federal Parliament after being chosen to fill the spot vacated by Liberal senator Arthur Sinodinos.

Key points: Jim Molan's pre-selection bid had the backing of Scott Morrison and John Howard

Jim Molan's pre-selection bid had the backing of Scott Morrison and John Howard He lost his Senate seat in May after being relegated to an unwinnable place on the Coalition's NSW ticket

He lost his Senate seat in May after being relegated to an unwinnable place on the Coalition's NSW ticket He previously served as chief of operations for the Coalition forces in Iraq

Mr Molan defeated Insurance Council of Australia executive Richard Shields for the position at a meeting in Sydney on Sunday.

The former senator said he was "very humbled" by what he described as a "decisive victory" and thanked Prime Minister Scott Morrison and former prime minister John Howard for their support.

He also confirmed he had agreed not to seek pre-selection again when his term expires at the next election.

"It is two-and-a-half years, that's a long time. Should I want to serve this country and serve the state of New South Wales after that, I have given my word that I will not run for the Senate again," he said.

"But I have a lot of things I want to achieve and I should be able to achieve them in the next two-and-a-half years."

Mr Molan's pre-selection comes six months after he lost his Senate seat at the May election, having been relegated to an unwinnable place on the Coalition's New South Wales ticket.

He said he had no regrets about a "below the line" campaign which sought to get him re-elected, angering some within the Coalition.

"I thank the people of New South Wales who voted for me in great numbers. They voted for an endorsed Liberal Party candidate who worked in accordance with the rules of this great party," he said.

"I thank them for that but what I'm doing now is looking to the future. I am not looking back, I'm looking to the future."

Senator Sinodinos is departing Parliament for Washington to take up the role of Australia's ambassador to the United States.

Mr Molan was sworn in as a senator for NSW in February 2018 after former deputy Nationals leader Fiona Nash was caught up in the dual citizenship scandal.

He previously served as Chief of Operations for the Coalition forces in Iraq and was the co-author of the Government's Operation Sovereign Borders policy.