The Government department that regulates the building industry has no record of instructions that the minister responsible for the sector says he made to avoid a possible conflict of interest.
Maurice Williamson said in February that he had "instructed officials" he would withdraw from discussions that could create a conflict between his roles as building and construction minister and director of Holyoake Industries, a large air- conditioning and ventilation company.
"On becoming a minister, I instructed officials that I would not receive papers on, and would withdraw from, discussions about heating and ventilation because of my directorship of Holyoake Industries Ltd."
However, in response to an Official Information Act request for a copy of the minister's instruction, the Ministry of Building, Innovation and Employment, which includes the former department of building and housing, said it "does not hold any information covered by this request" and that it was transferring the request to the Department of the Prime Minister and Cabinet.
Williamson's office has refused to answer questions on the issue, or make the minister available for comment.
A spokesman said Williamson was referring to the Cabinet Office when he referred to "officials". However, he has refused to release any correspondence to back the claim.
Labour deputy leader Grant Robertson said the ministry statement showed Williamson's statement was "misleading and meaningless" because the officials who would give advice on matters related to building and construction appeared to know nothing of the instruction.
"If the officials who are providing that advice don't appear to know there was any withdrawal from the minister, then clearly it's not happening," said Robertson, who has called for Williamson to step down from one of the two roles.
It was not the job of ministers to instruct the Cabinet Office, he said.
"He doesn't instruct them, that's not his job, they tell him [what to do].
"The prime minister can instruct those people, but not him."
The Cabinet Manual allows for ministers to hold outside directorships, but it requires the explicit approval of the prime minister.
Answers to written parliamentary questions from Williamson to Robertson show it was more than six months after the 2008 election before such approval was given.
John Key has dismissed questions about Williamson's dual role.
"He has done what was expected of him, and I am satisfied of his ability to manage any potential conflict," the prime minister said in a written answer to a parliamentary question.
In February, Key insisted the situation was no different to the finance minister being a taxpayer, or a farmer being in charge of agriculture.
Robertson said the primary industries minister did not make decisions that would affect individual farms in the way the building and construction minister could influence those sectors.