Palmerston North retiree Bruce Thompson made over $140,000 investing in cryptocurrencies over the past year.

A pensioner stands at an airport security checkpoint with a backpack full of gold bullion, a knife and false beard.

Why? To understand that, you need to know a little about Palmerston North retiree Bruce Thompson and why he's so excited about "cryptocurrency".

A former IT worker, and early adopter, Thompson has followed the rise of digital currency with avid interest.

SUPPLIED People from around the world trade in cryptocurrencies through online exchanges such as Poloneix.

​He's made $146,000 since he started dabbling with cryptocurrency in June 2016, $108,000 of that in the past month and a half.

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Cryptocurrencies are digital currencies that use encryption to secure transactions and control how new "coins" are made. Bitcoin is the most famous, but there are others, such as Ehtereum and Dash.

Each "coin" has a unique computer-generated number, and only a limited amount can be created. They can be used anonymously.

In mid-March, Thompson read about a major international bank buying Ethereum, and saw the currency's value start to tick upwards in the online exchanges.

He realised it was about to skyrocket. So he made a snap decision to dip into another investment he'd made over the years – his gold bullion.

That's how Thompson came to be standing at a checkpoint in Wellington Airport, with a bemused security guard giving him the side-eye.

The problem was his gold was in a safe deposit box in the State Insurance building in Wellington, and the vault was closing in only three hours.

He snatched up a backpack and dashed to the bus stop in Palmerston North, and arrived in the capital with an hour to spare. And he needed every bit of it to navigate the vault's tight security, so he sprinted three blocks through the crowded central Wellington streets.

Soon he was back on the street with his backpack $103,000 in gold bars heavier, and was ready to fly to Auckland to cash it in.

But in his rush to the vault, he'd never checked to see what else was in his backpack.

The knife was from a hiking trip, and the fake beard was part of a costume for an amateur musical production. It took some explaining, but he was eventually on his way, minus the knife.

"It was all a bit of a risk, but it has paid off hugely for me."

Thompson said he wanted to share the wealth, so he made a starter's guide to buying cryptocurrencies to make it a bit easier for others to seize the opportunity.

"When you make $140,000 basically overnight, you feel a bit guilty.

"You think, how can this just be happening to me? I should tell everyone else."

He said investing in cryptocurrency was a solid gamble, because it changed money and banking the same way digital technology was changing so many areas of life.

Thompson said people shouldn't invest with money they couldn't afford to lose, and they should practise with $10 until they got a handle on the market.

"I could be fatally wrong. But I've got enough put aside, I own my own house, and I'm a pensioner ,so I'm all right."