Mr Davis 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".
At the time, no New Zealand agency had received a formal request for assistance from the US in the Dotcom case.
Mr Williamson's office initially refused to say whether he had been briefed about the email by Mr Davis. Today, Mr Williamson responded to an urgent Official Information Act request and revealed he had been told last Friday by a senior Customs official about the "brownie points" email. It was the same day the Herald began making inquiries.
"It is an operational matter that is being dealt with by Customs."
But Customs continues to refuse to make any comment on the issue, saying only it is allowed to pass certain information to other countries.
Mr Browning said the minister appeared to be using "operational issues" to avoid getting involved in an incident which raised concerning questions.
"There needs to be some very clear explanations. Greg Davis should be held accountable. From that email, that appears to be something that is potentially even illegal that is being suggested."
He said Customs should take note of the statement made by Prime Minister John Key when he opened the Integrated Targeting Operations Centre, just weeks before Mr Davis sent the email.
At the opening, Mr Key said: "Anyone who is innocent has nothing to fear."
Mr Browning said Mr Key's view raised questions about Customs' silence - and its refusal to explain Mr Davis' actions.
He said it showed Mr Williamson needed to be involved. "What it the minister's explanation for Greg Davis' casual approach to the privacy of a New Zealand resident?"