A listening device has been discovered during sweeps for bugs at government ministers' homes.

The searches, by Government Communication Security Bureau (GCSB) staff, were done amid fears surveillance devices had been planted ahead of the November election.

Government sources told the Sunday Star-Times that ministerial homes and offices have been swept, sometimes secretly, and at least one listening device found.

The partner of one senior minister told the Star-Times that GCSB officials used high-tech equipment to scan their home for bugs. Nothing suspicious was found but the agents confirmed other recent searches had been positive, but gave no further details.

GCSB ensures politicians are free from intrusion, and staff inspect New Zealand embassies and offices around the world.

Electronic devices that are recovered are sent to the bureau's "Black Museum" in Wellington.

The bureau's scanning operations account for about 10% of its work and are one of its few publicly acknowledged tasks.

Prime Minister John Key's office refused to discuss the latest find. "We really don't talk about this type of stuff. We don't comment on security matters," chief-of-staff Wayne Eagleson said.

"It's been the practice of successive governments not to comment. Once you start, where do you stop? We don't consider it in the public interest to talk about these issues."

Police Minister Judith Collins was also tight-lipped. "It is not something I can discuss."

Diplomatic Protection Squad Head Inspector Terry O'Neil said he had no involvement in bug sweeps and directed inquiries to the GCSB.

In 2004, Maori co-leader Tariana Turia's ministerial house, which had also been home to Labour MP Colin Moyle, Progressive MP Matt Robson and National MP Murray McCully, was searched for bugs, and one phone was found to have been tampered with.

Act leader Don Brash said he instigated a number of sweeps in various government offices when he was National leader.

He said "a lot of soul-searching about espionage and techniques for accessing material inappropriately" was triggered when author Nicky Hager published his 2006 Hollow Men book, which contained private communications.

"There were some very strange goings on. It induced some paranoia," Brash said.

But he was surprised a current minister was the target of a bug. "We find it hard to believe people would be so sinister that they would bug ministerial offices, but we know it happens elsewhere, so maybe we are being naive."

Key has adopted a policy of not allowing cellphones, even switched off, in ministerial offices.

Former Labour prime minister Helen Clark also enforced the policy, which one of her former ministers said was adopted on advice from the GCSB.

HOW TO BE A SNOOP

A Sim card and $150 is all you need to become a spy – and a criminal.

Dozens of discreet listening devices are for sale online. One firm sells a Chinese device that allows its user to insert a Sim card and then call from any phone to listen in.

"Trying to find out the truth about what's being said behind your back?" the firm's ad says. "Now you can, and from anywhere in the world."

A director of the company said only "a couple" of the devices, which he described as "perfectly legal, as far as we know", had been sold.

The Crimes Act prohibits the use of any device that "intentionally intercepts any private communication". Offenders can be jailed for two years.

In 1995, prison officer Donald Trask was fined $2000 for possessing for sale a listening device he offered as useful for intercepting private communications, and police closed his Bug Shop on Auckland's Karangahape Rd.

Paragon Investigations private investigator Ron McQuilter said it was "shocking" how easy it was to buy listening equipment. His company offered bug sweeping and did around five searches a year, although he had found only one device during his entire career.