Mr Porter maintains he cannot and should not disclose the source of the money contributed through what he describes as a blind trust for his defamation case against the ABC.
Deputy Prime Minister Barnaby Joyce expects Mr Porter to one day return to the frontbench from the “corridor of the nearly dead”.
Mr Porter launched legal action over a story revealing a historical rape allegation, from a woman who had died, against a then-unnamed cabinet minister.
He named himself amid frenzied speculation before taking the ABC to court and settling the case before trial.
Mr Porter strenuously denies the sexual assault claim.
His acceptance of the mystery money and refusal to provide further details aside from assurances it was not from lobbyists or prohibited foreign entities, has fuelled opposition demands for a federal anti-corruption body.
Labor wants Mr Porter to reveal where the cash came from, return it, or leave parliament altogether.
Opposition Leader Anthony Albanese said there were many unanswered questions.
“Who provided this money? Why was it provided? How was it provided?” he said on ABC radio.
“Is there any conflict of interest? Does anyone who provided money have contracts with the government?”
Mr Albanese maintains Mr Porter’s resignation from the ministry does not alleviate disclosure obligations.
“There are real reasons why members of parliament have obligations to declare publicly any private interests that they receive,” the Labor leader said.
Mr Porter lashed the “Twitter mob” on Sunday, explaining he resigned because he would not pressure the trust to reveal who contributed to his defence anonymously.
“Ultimately, I decided that if I have to make a choice between seeking to pressure the trust to break individuals’ confidentiality in order to remain in cabinet, or alternatively forego my cabinet position, there is only one choice I could, in all conscience, make,” he said in a statement.
He vowed not to quit, which would have triggered a by-election, and said he planned to recontest his WA seat of Pearce.
Mr Joyce expects Mr Porter to eventually return to a senior role in the coalition.
“I bet you his electorate won’t resign from him though. He’s an incredibly astute politician, he’s incredibly capable,” Mr Joyce told Seven News.
“I’ll put money that we’ll see him back again.”
Employment Minister Stuart Robert described Mr Porter’s statement as “heartbreaking reading when you look at what he’s been through”.