Finance Minister Michael Cullen told a man who was worried about how many people were moving to Australia to "please join them".
The abrupt remark surprised Bay of Plenty businessman and National Party member John Middleton, who had emailed Dr Cullen a link to a report on nzherald.co.nz saying the number of people moving to Australia had hit a 19-year high.
He added a sarcastic attack on Dr Cullen, which questioned his performance.
But he didn't expect to be told to head over the Tasman himself.
"As far as I'm concerned he's a public figure in New Zealand and with a reply like that, it doesn't bode well," Mr Middleton said yesterday.
Dr Cullen said that people who sent him rude emails in the lead-up to Christmas could expect that sort of response.
"It's not much different from how members of the public would respond if they received correspondence like that," a spokesman for Dr Cullen said.
The exchange happened the day after Labour finally passed the Electoral Finance Bill - a process that had been particularly tense for the party.
Dr Cullen had also been in the spotlight two weeks before for calling National Party leader John Key a "scumbag" and "rich prick" across the debating chamber.
Dr Cullen's spokesman said the Finance Minister did not normally reply to emails but he did remember answering that one and did not think the response had been inappropriate.
Mr Middleton said he had emailed Dr Cullen before and had some "reasonably good" replies, but did not make contact regularly.
He said he was not an activist but had genuinely been concerned by the number of people moving across the Tasman, which had included some of his own family and staff in recent times.
"I'm a National Party man, no doubt, but at the same time I want to see the country go in the right direction, and I just see it's not," he said.
National Party finance spokesman Bill English argued the exchange showed Dr Cullen did not have the interest or patience to find a solution to the growing number of people moving to Australia.
"It's just a response that shows that he's given up caring," Mr English said.
"The trouble is that New Zealanders are leaving the country in planeloads, and Australians are coming here in kayaks."
In a later email Mr Middleton told the Finance Minister he did not intend to join others in crossing the Ditch, but "I'll go when you go". Dr Cullen wrote back: "I love it here".