A suspected cocaine shipment believed to have a significant street value was discovered by chance as an empty shipping container was being repaired in Christchurch.

Three large packages, each containing a white powder, were found by staff at the Lyttelton Port Company (LPC) city depot in Woolston about 2.30pm on Monday.

Investigators believe it is a shipment of cocaine, which has an average street value of about $400 a gram.

Large quantities of the addictive, Class-A controlled drug are rarely found in New Zealand and police and customs are now investigating the container's movements and destination.

They are tight-lipped about details of the discovery and will not reveal the quantity of cocaine seized.

Lyttelton Port of Christchurch chief executive Peter Davie said LPC staff were repairing a refrigeration unit on an empty shipping container when a suspicious looking package containing white powder fell to the ground.

Two similar packages were found and police were called.

The container was not in a customs controlled area at the time, he said.

"There are literally millions of containers that come into the country every year. If people go to the nth degree to stow things it's difficult to track down.

"If we see something suspicious then we hand it over to the relevant authorities. We are not actually the front-line agency looking for these things. Our staff have done a great job with this one."

Davie said he did not know where the container came from or where it was destined. He could not recall the last significant drug seizure at the port.

Detective Senior Sergeant Tony Hill said it was not yet known whether the cocaine was bound for the domestic or international market. Seizures of the drug were "relatively uncommon" in Canterbury.

The use and sale of cocaine had not come to the attention of the district's police in the past year, he said.

Customs declined to comment while the investigation was ongoing.

New Zealand Drug Foundation executive director Ross Bell said the country's cocaine use was modest, "because we're not seen as an important or lucrative market".

Most significant quantities of cocaine seized in New Zealand were bound for Australia.

"The rarity of [cocaine] is displayed in its price. It's an expensive drug," Bell said.

"It kind of has a reputation as being a bit of a party drug and harmless, but that's not always the case. People do get hooked and people do die from it."

Figures provided by police show authorities have seized 33kg of cocaine in the last six years.

Six people were arrested after customs found 3kg of 80 per cent pure cocaine inside the lining of a man's suitcase at Auckland Airport in December 2011.

The shipment, with an estimated street value of $1.2m, came from Mexico via Los Angeles.

Peru, Colombia and Bolivia are the world's leading producers of coca, the plant used to make cocaine.