Barnaby Joyce has been cleared of misusing taxpayer-funded travel expenses but investigators say they had to take his word that he was working.
The former deputy prime minister claimed at least $16,000 in taxpayer refunds for almost two full months he spent in Canberra last year when parliament wasn't even sitting.
But he has been cleared of misusing his taxpayer-funded travel to spend time with his adviser-turned-partner Vikki Campion.
An independent investigation found a "substantial change" in the amount of time Mr Joyce spent in Canberra.
Mr Joyce spent 58 nights in Canberra in 2017 when parliament wasn't sitting - compared to just 12 nights in 2015 and in 2016.
The Independent Parliamentary Expenses Authority said it had to take Mr Joyce at his word that he was telling the truth about his Canberra stays.
"Only Mr Joyce can determine that his overnight stay was primarily occasioned by that official business that Mr Joyce has identified," the authority said in its report on Thursday.
Mr Joyce quit as deputy prime minister in February after it was revealed he had left his wife to have a baby with Ms Campion, who had worked in his office as a media adviser.
The authority launched an investigation out of concern Mr Joyce might have misused his travel entitlements to be with Ms Campion.
Mr Joyce argued his ministerial workload significantly increased in 2017 when he took on the resources and northern Australia portfolios.
Ms Campion had to repay $100 in cab fares after she fell sick on a trip and spent three nights recovering with her family in Sydney.
But she came out ahead after the audit also identified a $978 travel claim she was entitled to that she hadn't been paid.
MPs got either $276 or $285 for every night they spent in Canberra in 2017, meaning Mr Joyce claimed at least $16,000 to be in the capital when parliament wasn't sitting, plus extra for the nights he was acting prime minister.
Michael McCormack, who replaced Mr Joyce as Nationals leader and deputy prime minister, said the investigation had shown there was nothing untoward.