Deputy Prime Minister Bill English is calling on the Labour Party to definitively declare whether it has ever received any donations from businessman Donghua Liu.
Labour has been critical of the National Party's links with Mr Liu, but the New Zealand Herald has reported that Mr Liu donated $15,000 to Labour in 2007.
Labour says it has no record of any such donation, but Mr English said on Tuesday it looks like the party has something to hide.
"So far you've had tricky and conflicting answers from David Cunliffe. I think it's time he answered some straight questions about this donor - particularly since he was so publicly critical of this donor."
Mr English said all parties are legally obliged to keep a record of their donors.
Labour leader David Cunliffe said that National is simply mud-slinging.