National is considering going to the police about the dealings of Labour MP Taito Phillip Field.
It might also alert other Government agencies - including the tax department - to accusations that the MP used "slave labour".
In a clear sign that Mr Field's time in the spotlight is far from over, National yesterday seized on details of a conflict of interest inquiry which suggest a Thai man was underpaid for working on properties the MP owned.
That, and other questions raised in the corruption inquiry by Auckland QC Noel Ingram, have led National to write to Parliament's Speaker and request that the privileges committee look into Mr Field's actions.
The party also said yesterday that it might take further steps to establish whether Mr Field used "slave labour" on several of his properties in return for immigration assistance.
"We've got to get our minds around the full implications before we make any decisions about putting matters in the hands of the police," National's immigration spokesman, Lockwood Smith, said.
"I suspect eventually we may - that is definitely an option open to us."
Other agencies National could alert include Inland Revenue.
Mr Field claims Dr Ingram's inquiry vindicated and exonerated him.
The Mangere MP was cleared of having conflicts between his private interests and his role as a minister.
But the inquiry raised serious questions about the appropriateness of his conduct as an MP.
The inquiry report said a Thai man did painting on several of Mr Field's properties for below market rates.
In one case, the inquiry found, the man was paid $350 for work which had a reasonable market rate of $1470.
There was an inference the man had worked out of gratitude or "some sense of obligation" for immigration assistance from Mr Field.
The report said Mr Field appeared to know the identity of the man who did the work, and - on the basis of a receipt - the amount he was paid.
There is also doubt over work done on Mr Field's house in Samoa by four other Thai people, who refused to talk to Dr Ingram.
Parliament's privileges committee can require witnesses to appear for questioning - something Dr Ingram couldn't do and which he made clear he thought had hampered his inquiry.
Speaker Margaret Wilson is expected to make her decision about involving the committee with little delay.
Asked last night about underpayment of people who worked on his properties, Mr Field put distance between himself and the painters.
He said he paid a woman who organised the work, and she hired the painters.
"It was like a contractor. We had a job to do, we gave them the job, and we paid the bill," he said.
"They weren't significant jobs. They were small paint jobs."
Asked if the payments went through correct tax channels, Mr Field said: "That's between her and her company - I think she has a company".
Dr Ingram's inquiry shows that the woman who organised work on some of the properties was very close to the Field family, and looked to Mr Field's wife as "like a mother".
Mr Field told the inquiry the woman also helped establish a Thai branch of the Labour Party in Mangere.
National wants a commission of inquiry into Mr Field's actions, but Prime Minister Helen Clark yesterday continued to resist those calls.
She said in Parliament that Dr Ingram's inquiry had cost $500,000, and "I do not consider further expenditure of public money warranted".
* Painter paid: $1500.
* Market rate: $5300 to $6300 for whole contract, or $4400 to $5100 for labour only.
* Painter paid: $750
* Market rate: $2200 to $2400 for whole contract, or $1700 to $1900 for labour only.
* Painter paid: $350
* Market rate: $1470 for labour only.