Federal Labor is concerned an "erratic" Barnaby Joyce could one day be the acting prime minister.
The deputy Nationals leader revealed on Thursday he is preparing to throw his hat into the ring to take over from Warren Truss.
Mr Truss is weighing up his future in federal parliament before the next election and could soon announce his retirement.
"It's a job I can do, but I'm not in a rush," Mr Joyce told Fairfax Media.
"I've had shadow portfolios, been leader in the Senate, deputy leader of the party, been a successful minister. That's what counts."
But Acting Opposition Leader Penny Wong raised concerns about Mr Joyce's ambitions.
"Barnaby is very entertaining, but he is erratic and that's the last thing you want in someone who is one step away from being the prime minister and who is called upon to act as prime minister," she told reporters in Adelaide.
"We all remember Barnaby being shadow finance minister. That was certainly entertaining, but if he can't hold down that job, I don't know how you hold down the deputy prime ministership."
Deputy Liberal leader Julie Bishop says the Nationals' leadership is a significant decision for the party.
"Barnaby is the deputy leader and is doing a very fine job," she told ABC radio.
"But the question of the leadership of the National Party, given that it brings with it the deputy prime ministership, is obviously a significant matter for the National Party to determine."
Ms Bishop described Mr Truss as a dear friend and good colleague, and said he should be allowed to make a decision before a debate begins about his successor.
Former deputy prime minister Tim Fischer told AAP he believes Mr Joyce was "well-experienced" for the job, but that it was a matter for the federal Nationals.
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