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Gillian Rodgers, 74, dug out her old suitcase on Saturday for a trip to Blenheim and found a tightly-packed bag of white powder, about the size of a packet of cigarettes.
"I was clearing it out and I saw this packet of white stuff and I thought, 'wow what's that?' I thought it was something to keep the suitcase dry. It was like a plastic packet, about three inches by two inches, and it was solid, like a cushion."
She had padlocked every pocket on her bag except the one in which the drugs were found, she said.
Not knowing what the package was or where it came from, Ms Rodgers took it to the Albany police station to be examined.
Police have praised Ms Rodgers and started an investigation.
"I couldn't believe it when the policewoman rung me back and said it was [amphetamine]. She said it was $50,000 worth.
"I didn't for one minute expect it to be drugs even though we were joking about it that it might have been."
Ms Rodgers last used the suitcase four years ago when she made a month-long cruise around Australia with a friend before flying home to Auckland from Sydney.
She suspects someone put the drugs into her suitcase pocket and that she was being used as mule to get the package into the country.
"I'm baffled. Of course I didn't see anybody put it in there. The only time I was not in my possession of my luggage was when it was on the carousel and when I put it on the X-ray machine.
"I still can't believe it was drugs. I wonder if they were going to try to retrieve it. I'm sure they would have wanted it back. I wonder if they followed me."
It was only luck she wasn't caught with the drugs at Auckland Airport, she said. But the discovery over the weekend raised questions about the screening process at the airports on both sides of the Tasman.
"I could've been picked up at the airport. Or if I'd gone through somewhere like Bali or Thailand with that in my bag ... terrifying."
Customs spokeswoman Nicky Elliott said the airport protocols were sound.
"There is no way to determine where and when the drugs were placed in the suitcase," she said.
"This could have happened anytime since she last used the suitcase."
She said Customs processed about 10 million travellers a year, and all were screened through risk assessment and profiling.
"If there is no alert, the passenger is not stopped unless as part of routine screening procedures.
"We are confident this system works well and that the border is fully protected."
North Shore area commander Inspector Shanan Gray said Ms Rodgers did the right thing by bringing in the "unusual" package.
The owner of the 55g of amphetamine - a Class A drug - could face a maximum life sentence, if found and convicted, Mr Gray said.
"We are investigating. If we can establish an owner of the item then we'd look at prosecution. We may look at Customs records, inquiries with the cruise ship company ... there's multiple lines of inquiries we can exhaust before we put it to bed."
Flight Centre New Zealand's general manager of retail, Sue Matson, said travellers should remain vigilant on holiday.
"Lock all zips and clearly label your bags, don't carry items for strangers, and, for extra safety, you can always get your bag wrapped at the airport.
"If something doesn't feel right, it probably isn't."