The US' Libertarian Party has nominated former New Mexico Governor Gary Johnson as its presidential candidate for the second time.

Johnson, 63, won the nomination of the US' third political party on the second ballot at the party's convention in Orlando, Florida, on Sunday.

He defeated Austin Petersen, the founder of The Libertarian Republic magazine and anti-computer virus company founder John McAfee.

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Johnson told the delegates during his acceptance speech that his job will be to get the Libertarian platform before the voters at a level the party has not seen.

"I am fiscally conservative in spades and I am socially liberal in spades," Johnson told the AP news agency.

"I would cut back on military interventions that have the unintended consequence of making us less safe in the world."

On fiscal matters, Libertarians push for reduced spending and taxes, saying the federal government has become too big across the board. Johnson proposes eliminating federal income and corporate taxes and replacing those with a national sales tax.

He would reduce domestic spending by eliminating the Internal Revenue Service, the Commerce and Education departments, the Food and Drug Administration and the Drug Enforcement Administration.

Freedoms high on agenda

On social issues, Libertarians generally support abortion rights, gun rights, same-sex marriage and drug legalisation, saying people should be allowed to do anything that does not hurt others.

Johnson served as New Mexico's governor from 1995 to 2003 as a Republican after a career as the owner of one of that state's largest construction companies.

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After failing to gain traction in the Republicans' 2012 primaries, he changed his registration to Libertarian shortly before running for that party's nomination that year.

He won the nomination and got just short of 1 percent of the general election vote against President Barack Obama and Republican challenger Mitt Romney.

For Johnson to make a serious run this year, he needs to qualify for the presidential debates. To do that, he must average 15 percent in five recognised polls.

He hopes that is doable because the Republicans' Donald Trump and the Democrats' Hillary Clinton are both seen unfavourably by a majority of voters, according to recent polls.