Deputy Labour leader Michael Cullen says National Party leader John Key acted unethically after disclosures about the number of TranzRail shares he held while making comments on the railways.
The Government has previously accused Mr Key of a conflict of interest, saying his family trust owned shares in TranzRail while he asked Parliamentary questions as National's associate transport spokesperson in 2002 and 2003.
Reports at the time said his shareholding was limited to 30,000 shares - a figure not disputed by Mr Key.
Dr Cullen says Mr Key actually held 100,000 shares in TranzRail and has misled the public.
Mr Key says the number of shares held by the trust varied and he sold them when he realised TranzRail was to become a major political issue. He says the result was a substantial loss.
Dr Cullen told Morning Report that Labour is not operating a smear campaign, but is simply trying to get the facts out in the public.