That's strange," a friend says as we approach an empty shop front in London's Covent Garden. "There was a bar here last week, I'm sure of it."

Indeed there was. A happening place it was, too. But this week, it's gone; the building stripped bare but for a couple of old posters and a "for lease" sign hanging in the window.

No, this isn't another story about recessionary doom and gloom. The bar was only ever intended to be a temporary fixture. A limited hot spot of drinking and debauchery that shut shop just weeks after it opened, despite its flourishing popularity; otherwise known as a pop-up bar.

Based on the principles of scarcity and exclusivity, pop-up bars are appearing everywhere these days, having captured the ever-fickle imagination of Generation Y.

You only have to walk down Auckland's Ponsonby Rd or around the Viaduct to see just how mercurial such tastes can be. Every month, new signs appear as establishments change hands and change names, trying to lure a lasting, loyal client base.

But pop-up bars aren't interested in longevity. They burn bright and fast - and then they're gone.

"It's like a fleeting summer romance," explains Simon Pound, one of the men responsible for bringing pop-up bars to Auckland. "It's got that sense of adventure and abandon."

Last month, Pound and his two business partners, marketing consultant Hadleigh Averill and Dirty Records' head Callum August, opened a pop-up at the Birdcage in Freeman's Bay. The bar, which had capacity crowds throughout the week it was open, will move about the city in coming months, appearing at various venues.

For Pound and his partners, the idea of a pop-up was originally a stepping-stone to opening a permanent bar one day. But they soon discovered it had a number of advantages over a permanent set-up.

"Bars often have natural lives. Sponge [on Ponsonby Rd], for example, used to have lines outside morning, noon and night. [It was] pumping the whole time. Now it's not."

And pop-ups don't just appeal to revellers' short attention spans. They keep things fresh and exciting for the owners and employees.

"Bars are like groundhog day," says Pound, who spent several years working as a barman at Auckland's now-defunct Crow Bar. "If you run a bar, you turn up everyday, do the same set-up procedures, you serve drinks, you clean up and you go home. A lot of great experiences are had but that's why hospo has such a high turnover ... It's monotonous, hard work. The beautiful thing about pop-ups is they have this natural short life and you're not tied to something forever. People are interested because they're new and exciting."

Simon Jamieson, general manager of Sky City Hotels Group, was the man behind one of Auckland's first pop-up bar, the Sky City Pacific Bar, open for three weeks in February during the Louis Vuitton Pacific Series.

Although that pop-up was a one-off venture designed for a specific event, it showed just how successful the concept could be. For three consecutive weekends, the bar played host to thousands of people as they watched the yachts return to the Viaduct Basin and celebrated late into the night.

"Perishability is the big thing," says Jamieson. "Done properly, that's what piques interest. The texture of these bars and restaurants is the fact that you want to say you've been, you tried it and you were there. So you're in a unique club of people who managed to get there."

Pound and his partners have taken that idea one step further, issuing special pop-up membership cards at their last event. They're designed to give members special privileges and help build a following that - if all goes to plan - will move with their pop-up bar wherever it goes.

"Bars are really about the people you're around," says Pound. "With pop-up, you know what type of people will be there and what type of night it will be." But there also must be something in it for the punters.

"Novelty is a big thing but you've got to give them more than novelty," says Pound. At the pop-up launch party, the organisers worked with a liquor company and offered free vodka throughout the night. Which brings us to the real crux behind pop-up bars.

Despite their seeming spontaneity, they still require enormous amounts of planning, energy and expense. The process to get a liquor licence and resource consent for a bar is the same whether it is open for three months or three decades. So why go to all that effort? Surely it's not a viable business model? The simple answer is marketing.

Pop-up bars aren't about selling alcohol and turnover. Rather, they are marketing vehicles, used to expose certain audiences to a product. They provide an experience and a memory, which will long outlast any conventional advertising.

In Australia, New Zealand winery Stoneleigh has designed a pop-up bar in a shipping container, which can be moved from city to city to introduce people to their product. Jack Bedwani of Australian marketing agency The Projects created the Stoneleigh Lounge for the wine company. He sees the pop-up bar as the perfect way to target media savvy, "early adopters".

With live acoustic performances, wine tasting sessions and other special events, the portable wine bar has proven a massive success for Stoneleigh and Bedwani. They are already receiving requests for 2010 and 2011 and Stoneleigh's New Zealand headquarters are in talks to get their own version here.

"The consumer reaction has really surprised us because it's just something they have really embraced," says Bedwani. "Full credit to Stoneleigh, because they did take a risk on this. It was a significant investment up front for something that wasn't particularly tried or tested."

Likewise, beer giant Fosters is behind one of Sydney's most popular pop-up bars, The Pond, which was created by members of the local community who took part in a series of working bees to help renovate and restore the derelict building. Designed to market Fosters' Pure Blonde beer range, The Pond was created with a deliberate emphasis on purification and improvement.

To that end, the designers took over a 138-year-old building in Darlinghurst and enlisted the local community to help convert it into a popular beer garden. Barrie Barton, creative director of Right Angle Studio, designers of The Pond, says pop-ups cannot simply rely on a limited time frame to be successful.

"There's the full spectrum that ranges from great pop-up to bad pop-ups. The bad pop-ups are the ones that use this idea of a limited period of time to just sell an average product and compress and condense consumer expectation into a very short period of time.

"That's a really simple trick and it is just a trick. The more creative and impressive pop-ups have a great concept and when you put a parameter around that thing and make it for a limited time, then that concept does become more desirable."

Scheduled to run for just three months, The Pond has definitely captured Sydneysiders' attention. Night after night, the restaurant and beer garden is booked solid and there are plans to extend it throughout summer.

"To be quite honest, it's been a lot more popular than we ever expected it would be," says Barton. "People are emotionally connected to it and we have people coming back once or twice a week."

It is a perfect example of how experiential marketing can work to everyone's benefit. And it's an idea Pound and his partners hope will work here. "Everyone wins," says Pound.

"The pop-up inside a venue increases the clientele. The clientele win because they get privileges and the company that pays for it all wins because they get the exposure."

The question is: will the fickle tastes that were so drawn to the idea of pop-up bars tire of them just as quickly? Bedwani doesn't think so. He says we have seen only the first incarnation of pop-up bars and he believes they will continue to develop and thrive for years.

"I think liquor licensing laws are going to have to progress a little further for them to be a commercial reality, because it's still not an easy proposition for anyone to undergo. To get a licence that can move is still a challenge. But I think if the councils start to embrace these things more, as I'm sure they will, then I think definitely mobile bars will be, somewhat, the way of the future."