Labour has accused National leader John Key of lying over his ownership of Tranz Rail shares after revealing he failed to completely disclose his interests.
Labour research has found that Mr Key owned 100,000 Tranz Rail shares rather than the 30,000 owned by a family trust he previously admitted to.
He bought 50,000 of the shares in his own name while he was an MP and on the transport and industrial relations select committee.
The shareholding was first raised by Prime Minister Helen Clark in July, when she accused Mr Key of having a conflict of interest for having the 30,000 shares in 2003 while criticising the Government's bid to buy the rail operator back.
Mr Key admitted to the 30,000 shares and said he sold them when he wanted to speak out about the Government's plans. But Deputy Prime Minister Michael Cullen last night released share register information showing the 100,000 shares and said Mr Key "failed to correct reports" his shareholding was limited to the 30,000 shares.
"John Key lied because he had something to hide," Dr Cullen said.
Dr Cullen said the new information showed Mr Key "was commenting publicly on Tranz Rail, meeting with bidders for the track, and pursuing the release of relevant information while an undisclosed shareholder".
Dr Cullen said the 50,000 shares Mr Key purchased should have been disclosed in line with Parliament's standing orders.
Labour will now use the non-disclosure as a major platform of attack against Mr Key.
"Mr Key has spent a lot of time over the last few weeks attacking the character and credibility of others," Dr Cullen said. "He said people should not have to wait for direct questions to provide relevant information to the public."
In an interview with One News last night, Mr Key faltered when asked about the shares, initially saying his family owned 25,000-50,000 shares, then shifting to "sometimes 50,000, sometimes 100,000" then finally "yeah, sorry, it was 100,000 in total".
Asked why he had only ever admitted to the 30,000 shares owned by the family trust, Mr Key said: "No one's ever asked me the number I owned."
In a media statement he said: "The number of shares held by the trust varied at different times.
"The shareholdings were managed on my behalf by my broker who had the authority to act on individual share parcels without referring back to the trust."
Mr Key said the "attack" by Labour was "further evidence they will run a desperate smear campaign against me".
Labour's research shows Mr Key's trust bought 30,000 shares in 2002 before he was an MP. The trust bought another 20,000 shortly afterwards.
Mr Key then entered Parliament and asked parliamentary questions about Tranz Rail, not disclosing his shareholding.
In May 2003, Mr Key bought the 50,000 shares in his own name.
He sold the shares in June 2003 in two 50,000 lots.