New Zealand First Party leader Winston Peters is due back before Parliament's Privileges Committee - a day after Monaco-based businessman Owen Glenn gave further details about a donation of $100,000.
Mr Glenn appeared in person on Tuesday, armed with telephone records, emails and an affidavit he says proves his version of events.
Prime Minister Helen Clark says Mr Glenn's evidence is disturbing, and she will keep a close watch on developments .
She says it's important Mr Peters has the right of reply, but she has not ruled out taking action against him before the committee completes its investigation.
She says she will be watching what Mr Peters has to say when he makes his third appearance before the committee on Wednesday night.
The Privileges Committee is considering whether Mr Peters should have declared Mr Glenn's donation toward the cost of an electoral petition in Tauranga after the 2005 election. Mr Peters claimed that National Party MP Bob Clarkson had overspent on his successful campaign.
Mr Glenn says it was Mr Peters who personally asked him for a donation, while Mr Peters maintains the approach was made by his lawyer.
Mr Glenn told the committee on Tuesday he had brought with him copies of emails and telephone records to back his version of events.
He said he rang Mr Peters in December 2005 to inform him that he agreed to contribute $100,000 to the New Zealand First leader's legal costs.
The businessman was asked by Labour members on the Privileges Committee if he was sure it was Mr Peters on the line. Mr Glenn replied he had no doubt that it was Mr Peters.
Mr Glenn presented the committee with a copy of an email he received from Mr Peters' lawyer Brian Henry shortly after the telephone conversation.
In that, Mr Henry said he was emailing to provide bank account details, following Mr Glenn's conversation with "his client".
Mr Glenn also presented the committee with an affidavit from his racing manager Paul Moroney, who said he was present at lunch in Karaka in January 2006 where Mr Peters thanked Mr Glenn for his "help" to him.
Mr Peters maintains he was unaware of the donation until July this year, after he was told about it by Mr Henry.
The Privileges Committee is continuing its hearings and will hear from Mr Peters again on Wednesday evening.
He has stood aside from his portfolio responsibilities, including Foreign Affairs Minister, but remains on a ministerial salary. His portfolios are being administered by the Prime Minister.