Labor has seized on Helloworld correspondence naming ambassador Joe Hockey, who foreign affairs officials say complied with pecuniary interest rules.
"Hockey owes me."
A bombshell letter has deepened the Helloworld scandal engulfing the federal government and Australia's ambassador to the US, Joe Hockey.
When diverted from his 2017 holiday in Europe to Washington to speak with Mr Hockey about the embassy's travel needs, former Helloworld employee Russell Carstensen asked his boss how the meeting was arranged so quickly.
Mr Carstensen has explosively claimed, in the letter tabled at Senate Estimates, that Helloworld chief executive and Liberal Party treasurer Andrew Burnes called in a favour to arrange the meeting.
"I asked Mr Burnes how could this be done so quickly he verbally advised me, 'Hockey owes me'," Mr Carstensen wrote to a Senate committee on Thursday.
"I found that 'owes me' comment strange in the circumstances but it's not an unusual term from Mr Burnes when talking about his business relationships."
Mr Burnes emphatically denied the allegations.
"Joe Hockey and I have been close friends for 20 years and it would be ridiculous to suggest I would say or imply he owes me anything," he told the Sydney Morning Herald.
"I did not organise any meetings between Russell Carstensen and Joe Hockey. Mr Carstensen's own email of April 24, 2017 shows that he organised the meeting with Mr Hockey and I was simply CCed on the email."
Mr Burnes said the ambassador was unhappy with his travel arrangements and he was keen to set up a meeting.
Mr Hockey is one of the top 20 shareholders in Helloworld.
In his letter, Mr Carstensen said he gave Mr Hockey travel option details that could have been used "as the basis of tender requirements".
He later advised Australian government officials he met with the ambassador.
"The feedback from the WoAG (Whole of Australian government) Travel team was that they were uncomfortable with the meeting," he said.
"I was advised later in a conversation that 'people' in DFAT were uncomfortable that Mr Hockey met me and that he was a shareholder of Helloworld.
"If I remember correctly, I did not know at the time of the meeting in Washington that he was a shareholder."
Initially, Foreign Affairs secretary Frances Adamson said she was confident Mr Hockey met the required rules in declaring his links to the travel company.
He declared an interest in the company prior to the meeting and recorded his financial stake in annual disclosures the following month.
"He has done what he needed to do in terms of declaring, taking steps, and subsequently removing himself from any involvement in procurement," she told the committee.
Ms Adamson later toned down this endorsement.
"There are a whole range of things you have raised today I think we need to think about and we need to provide answers to you," she told Senator Wong.
"I'm not in a position now to say that in all respects those actions have met the standards."
Labor is considering whether to sack Mr Hockey from his post if the party wins the next election.
"This is a Liberal government of the donors, by the donors, for the donors," Labor leader Bill Shorten told parliament.
Earlier this week, it was revealed Helloworld had not charged finance minister Mathias Cormann for family flights to Singapore, which were booked through Mr Burnes.
The senator has since paid for the flights.