Labour and the Greens have slammed a Customs proposal that it be given blanket authority to force people to disclose passwords to their computers and smartphones at the border.

Green communications spokesman Gareth Hughes said requiring people to disclose passwords to electronic devices without good reason would be "completely unacceptable" and went against common perceptions of privacy.

His Labour Party counterpart Clare Curran said it was "another step towards a surveillance society and should be strongly resisted".

KHAM Customs proposal forcing people to disclose passwords at the border has been slammed.

Customs is seeking the automatic right to require people to disclose passwords to their electronic devices when entering New Zealand.

Failing to do so without reasonable excuse should be an offence punishable by three months prison, Customs has suggested.

It said the power would be useful in helping detect objectionable material and evidence of other offending, such as drugs offences, as well as to verify people's travel plans.

Many travellers were now carrying electronic travel documents, such as tickets and booking details, on computers and smartphones, it said.

READ MORE: Customs seeks powers to get passwords

Hughes said the proposed power was an "affront to Kiwis' privacy" and unnecessary.

Customs could already access travellers' electronic devices if it got a warrant," he said.

"There needs to be a clear threshold and due process for law enforcement agencies accessing private electronic devices," he said.

"It's disturbing that at the same time that we find out that our Government has been spying on our Pacific neighbours, the Government also announces it wants to be able to look at everyone's private information when they enter New Zealand without a due process or threshold to determine if that person is even likely to be hiding anything."

Curran said she feared "such draconian laws" would stifle innovation from businesses that provided encryption services and could dissuade people from travelling to New Zealand altogether.

The proposal is one of several canvassed in a discussion paper on a review of the Customs and Excise Act that was released by Customs yesterday, none of which are official government policy.

Customs also wants its officers to be able to compel people to empty their pockets if asked. It said they did not currently have that power unless they had "reasonable cause" to suspect someone was hiding something.

The discussion paper said it was rare for people to refuse to empty their pockets when asked. "However, even a small proportion of people refusing to do so can present a significant threat to New Zealand."

Council for Civil Liberties spokesman Thomas Beagle said that under existing legislation, people could not be compelled to disclose passwords, but Customs had the right not allow electronic devices to be brought into the country if people would not unlock them.

Beagle said he believed that was "a good compromise". It could not be assumed people would always have the passwords to encrypted files on their computers or smartphones, he said.

"They would be implying people couldn't travel with encrypted data at all if they did not know the password for it."

Another change would let businesses hold their business records overseas, rather than in New Zealand. But Customs said that should only be with its prior approval.

The change would make it easier for "trusted businesses" to take advantage of overseas cloud computing services and would bring the department more into line with Inland Revenue's policies, Customs said.