A senior Customs official told colleagues in charge of highly personal information they could earn "brownie points" if they passed secrets to the FBI.
The offer was made by a Customs executive whose job was to oversee one of the organisation's most sensitive units.
Greg Davis was the operations manager of Customs' Integrated Targeting Operations Centre when it was opened in September 2011.
The unit prompted privacy fears when it opened as the first outside the United States to actively harvest massive amounts of information from everybody arriving at or leaving New Zealand.
Prime Minister John Key opened it saying: "Anyone who is innocent has nothing to fear."
Just weeks after it opened, Mr Davis wrote to Immigration NZ's intelligence unit, which also holds highly personal information, telling staff there of the FBI's interest in the internet tycoon Kim Dotcom.
In an email released under the Official Information Act, Mr Davis told an Immigration NZ intelligence officer that Customs' Washington liaison was after information about Dotcom.
He said "the FBI would be interested in anything we have on Kim Dotcom so any information we can proactively feed to them on him will buy you many brownie points".
Immigration NZ staff passed around the request, with one person advising they seek legal advice before passing any information to the FBI. A spokesman said no information was passed on but refused to say if that was because legal advice advised against doing so.
At the time of the request, no New Zealand agency had formally been engaged under legal assistance laws with the US. Dotcom's status at the time was the same as any other public citizen.
A spokeswoman for Customs refused to comment on the email. She also refused to say if any information was passed to the FBI by Customs.
She also refused to say whether doing so would have been legal - and again refused to comment when asked to clarify the law regarding sharing information with the United States.
Labour's Grant Robertson, who has followed the Dotcom case closely, said Customs should explain what occurred.
"Our actions should not be based on trying to earn brownie points in Washington. Our actions should be based on rule of law."
Greens' Customs spokesman Steffan Browning said it showed a willingness to pass information to the US when it was not justified.
"We are not an American state at this point in time - but some of our Customs, police and security bureaucrats are treating us as one."
A spokesman for Customs minister Maurice Williamson said the issue was "operational" and there would be no comment. However, he refused to say whether the minister was seeking more information or whether Customs had already briefed him on the issue.