Reports emerged on Friday One Nation's leader paid more than $800 a night to stay at the five-star Conrad London St James hotel.
The room was left empty for five nights while Senator Hanson flew to Italy, according to reports in the Sydney Morning Herald.
One Nation senator Sean Bell said he has no knowledge of the allegations, dismissing criticism of the trip.
"I can assure people ... when we go overseas, we do so making sure people get bang for their buck," he told News24.
The allegations come after One Nation's Fire the Liar campaign accrued $5 million from thousands of small donations in a only few months.
The funds raised will pay for billboards and broadcast advertising to "reach Australians that Albanese ignores", according to the One Nation website.
Senator Bell sought to assure donors the money raised through the grass roots fundraiser had been "cordoned off" and cannot be used for other causes, including international travel.
"All the money that was sent to that is being used on that issue," he said.
Senator Hanson has previously defended the overseas trip, which included a five-night stay at an Italian resort with mining magnate Gina Rinehart, insisting taxpayers did not have to pay a cent.
Increased scrutiny has been levelled at One Nation as the minor party has repeatedly out-polled the major parties.
Immigration Minister Tony Burke's last-minute decision to pull out of a National Press Club address has been claimed as a win for the minor party.
Mr Burke was expected to outline a new approach to immigration standards to meet the government's net migration target of 225,000 people.
Senator Bell said he welcomes the major parties coming to the debate.
"I'm glad we can all agree that One Nation was right about the need to end mass migration. Everyone agrees," he said.
"Now we're just arguing about the best way to do it."
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