Barnaby Joyce will return as Australia's deputy prime minister after a sudden leadership contest in the National party, the government's junior coalition partner.

Mr Joyce defeated Michael McCormack in a party vote on Monday.

It follows growing concern from some National MPs over their party's influence in climate policy.

The party, which represents farmers and rural voters, has 21 members in the governing centre-right coalition.

Over the past week, National party members voiced opposition to indications from the government that it is moving towards a 2050 net zero carbon emissions target.

At the G7 summit last week, Prime Minister Scott Morrison displayed growing support for the target.

Critics of Mr McCormack said he had not been assertive enough about the National party's stance within the coalition.

That set the backdrop for the leadership contest which Mr Joyce reportedly won by a narrow margin.

He is expected to push for changes in the coalition agreement.

A well-known character in Australian politics, Mr Joyce previously led the National Party from 2016 to 2018 but was forced to resign after public pressure over his extra-marital affair with a staffer.

"I don't want to dwell on the personal, except to say, hopefully one learns from their mistakes and makes a better person of themselves," Mr Joyce told reporters on Monday.

Analysis box by Shaimaa Khalil, Australia correspondent

Barnaby Joyce's return to Australia's second-highest office is significant not just because he makes his political comeback, but because of what it tells you about the mood inside the National Party.

There's significant ire among the party's ranks about the Prime Minister's increasing support of a net zero carbon emissions target by 2050. Remember though, Mr Morrison has so far refused to commit to the date. But the mere suggestion of him inching towards more serious climate policy is enough to anger his coalition partners. Many National lawmakers have been public about their opposition to the government formally embracing the target.

Mr Joyce will likely take a hard line on climate policies, making any small steps the government takes towards emissions reductions very tricky.

It's yet another reminder of how politically toxic the climate change debate is here in Australia. The Prime Minister is stuck between two difficult pulling forces - international pressure from strategic allies like the UK and the US for more robust emissions reduction targets, and a coalition partner that is firmly wedded to the country's fossil fuel industry.

The first is about Australia's standing in the world. The second is about the Prime Minister's domestic political standing with elections looming.