Minister for Northern Australia Matt Canavan "forgot" he was a member of the North Queensland Cowboys NRL club when the Northern Australia Infrastructure Facility gave the club a $20 million loan for a new training centre.
However Senator Canavan believes he has not breached ministerial standards.
The Nationals minister made the revelation while announcing he was quitting his portfolio to throw his support behind Barnaby Joyce in the party's leadership spill on Monday night.
The Northern Australia Infrastructure Facility - which sits in Senator Canavan's portfolio - offers up to $5 billion over five years in concessional finance to encourage private sector investment in infrastructure in northern Australia.
The federal government in March announced a $15 million grant for the facility.
However Senator Canavan said he had "recalled" in the past week that he signed up as a regional supporter for the Cowboys some years ago, which entitled him to a membership of the club. He had not declared that membership.
"I don't have any control influence over the football club itself but at the same time it's an interest I should have declared and I have not," Senator Canavan said.
Senator Canavan said he did not think it was a breach of ministerial standards as he had no power to approve loans - only reject them - under the North Australia Infrastructure Facility Act.
Nevertheless he had sought advice from the Prime Minister's office about whether this constituted a breach of standards.
His admission came off the back of fellow Nationals leader Bridget McKenzie's resignation, after Department of Prime Minister and Cabinet secretary Phil Gaetjens found she breach ministerial standards by failing to refuse herself when signing of on grants to gun clubs she had undisclosed interests in.
Senator McKenzie maintains she received no personal benefit from her connection the clubs, and it did not inform her decision making.
It's not the first time Senator Canavan has been caught unaware.
He blamed his mother for signing him up as an Italian citizen when it was revealed he was a dual national, which could have seen him thrown out of parliament.
However he managed to survive the High Court challenge due to Italy's complicated citizenship laws.
