Air NZ offers free upgrades ... but there's a catch

Supplied pix from Transformers III Dark of the Moon for Timeout.

Forget the comfy trackpants and Ugg boots. It's going to pay to be wearing clothes that scream "effortless LA cool" if you're flying across the Atlantic next month as our national carrier rewards the most stylish passengers with free upgrades.

In a promotion linked to Los Angeles Fashion Week, Air New Zealand has appointed a stylist to walk the terminals at London's Heathrow airport to give away free upgrades and indulgent treats to those passengers adorned in the most on-trend looks.

The promotion has even caught the attention of UK fashion bible Harper's Bazaar, which suggests taking the lead from English beauty Rosie Huntington-Whiteley, who is often snapped at airports after longhaul flights looking fresh and stylish, to land an upgrade.

How dressing well at the airport could get you a flight upgrade https://t.co/QduKaKCtSL pic.twitter.com/czIkmM48Bd

— BazaarUK (@BazaarUK) March 7, 2016

For those struggling in the fashion stakes, the Air New Zealand UK website includes a style guide from the woman who will be casting an eye over passengers.

Stylist and former fashion editor Laura Martin describes LA fashion as modern and accessible, which means anyone can play a part in it.

She writes current LA trends are distinguished by an effortless cool, laidback style largely driven by the weather.

She says it is also very diverse and influenced by the number of unique areas within the city that all have their own personality and style.

Her tips include noting the beach surfer culture features shorts, denim and jandals where "heels are a big no no!"

In comparison, Beverley Hills was more designer and polished and Hollywood was described as a "mixed zone".

She said as you headed to the hip areas of East LA, Silverlake, Echo Park, there was a huge hipster culture reflected in a love of hats and big moustaches where 70s vintage was one of the main vibes.

"Whether people are rockers, hippies, or skaters, in LA they describe their style as 'free'. It's cool, casual, eclectic, effortless -- basically anything goes and designers are tapping into it," she wrote.

But one element consistent across all LA style was accessories.

"Big hats and sunglasses are a must and for women these can often be accompanied by fashion jewellery, and the right handbag!"

The promotion, which runs between April 14-16, not only includes business upgrades but lounge access, pre-departure champagne and free pamper kits for a comfortable journey.