Happy birthday Kate Sylvester!

High-waisted 50s knickers and fake furs, red stripes and grey marles, the perfect sheath dress delivered each season in a new killer print - all of it strangely sexy but sensible at the same time. Kate Sylvester's style is so much a part of the framework of the way we dress ourselves now in New Zealand, it is hard to imagine there was a time when she didn't exist.

Has it really been 21 years? I should know, I guess, because I was there at the birth.

In 1993, my flatmate Arthur introduced me to Wayne Conway and Kate Sylvester, two of his old friends from design school days who had come back home from Paris to set up their own fashion label. They already had a name for it: it was to be called Sister.

They were beautiful, Wayne and Kate. Outrageously, fashionably skinny - not from Paris though, but from travelling home via a month in India. The pair of them had cheekbones like coat hangers and rangy frames. Wayne looked like a young Marlon Brando and Kate's good looks were saved from being too unbearably model-perfect by the fact that she had a nose that was slightly off-kilter. It had been broken in Paris when a man on the street got furious with his blonde girlfriend and, in a case of mistaken identity, punched Kate in the face. Kate didn't mind the welterweight look the nose gave her, and she loved to tell the story of how she had been felled to the ground and the ladies in a nearby dress shop had rushed out with armfuls of gorgeous Liberty prints, which they used to staunch the flow of blood.

Wayne and Kate's plan for world domination had a couple of flaws: they knew no one and they had no money.

I didn't have any money either, but I was a freelance writer so I did have a typewriter in the warehouse where I lived down on Quay St. Wayne used it to write a business plan and, incredibly, the stupid bank gave him an unsecured loan. So the money bit was solved.

Armed with some cash, Wayne and Kate rented a space in Kitchener St. Wayne went about designing and creating the space for their first store while Kate sewed every bit of her first fashion range herself. Meanwhile, I worked on generating some buzz and writing out a guest list for the opening.

The launch party, based on my contact book, had to qualify as one of fashion's strangest get-togethers: the guests included Paula Ryan, Maysie Bestall Cohen and Supergroove.

Kate remembers standing there and realising she didn't know a single soul in the room and speaking to no one. Which probably went a long way towards creating that cool aura of aloofness that has stayed with her ever since.

Wayne and Kate were too poor to rent a flat as well as a shop, so they built walls for a tiny bedroom and lived at the back of the Kitchener St store. This was wildly illegal, so they had to be able to hide the bed and pretend the whole place was a workroom whenever the landlord came around.

I'd like to say the launch party set them up for life, but it didn't. The store, filled with the clothes Kate had sewn, was a damp squib. No one was buying.

The bank had reached its limit on foolish-ness. Wayne did the sums and realised they were losing money they didn't have hand over fist. Serious talks were had and it was decided that if things didn't pick up by Christmas they would pull the plug. Hacked off with the whole thing, Wayne designed a T-shirt that summed up his mood. It was a photo print of a cool girl raising her middle finger with a line underneath that read: Sister - you know you want it.

The T-shirt, which was meant as a f***k-you to the ungrateful non-customers, sold like mad. They did reprints. The Christmas rush came and the rest of the clothes sold, too. They were on their way.

Like that Christmas T-shirt, many of the good things that happened to the brand came out of bad beginnings. When the landlord booted them out of Kitchener St they were forced to bite the bullet and pay big rents to relocate in High St - the first of many retail moves that firmly established the firm as a serious member of the fashion mafia. Then, when the iconic Belinda Seper bought the range for George's department store in Melbourne, it appeared to be great news - until Australian streetwear label Stussy Sista saw their "Sister" clothes on the racks at George's, decided the names were too similar, and took up legal proceedings for breaching brand copyright.

Wayne and Kate met me for dinner at Stella with the lawyer's letter from Stussy Sista. They were realists - they didn't have the money or time to fight. If they were going to succeed in Australia, they had to change their name. But Kate didn't want to call the brand Kate Sylvester. She said it was like being at school camp with her name stitched inside everyone's jumpers. In her mind she was Kate Sylvester - the clothes were something else. Wayne and I talked her round. The argument was simple. All the biggest labels in the world were real names: Chanel, Gucci, Givenchy. Suck it up.

Wayne designed a new label and Kate Sylvester was born.

Wayne's quiet design strength was always a key part of the brand's story. He designed all their stores with help from friends, architects Bianca Pohio and Cameron Pollock, and furniture maker David White. And each season's range features his designed prints. He's also the one who conceives and executes the runway shows.

Kate, meanwhile, was figuring out how to design a fashion collection. At first she just made "clothes she liked". Then she began to see the bigger picture and began to learn how to put together pieces of the puzzle so they made a whole. She developed the idea of a theme to anchor her vision, because otherwise where was the intellectual challenge of making clothes every six months?

Her themes grew out of the stuff that was her life. There were books, of course: the Catcher in the Rye collection (that blue ice-skating dress is taken directly from the one Sally wears in the book on her doomed date with Holden Caulfield), the buttoned-up tweeds and handknits of Love in a Cold Climate inspired by the mad Mitford sisters, Brighton Rock referencing Graham Greene's novel and This Charming Man - inspired by foppish old goose Marcel Proust.

Later on, her eyewear collections would be named like a recommended reading list - Sylvia (Plath), Harper (Lee), all the Brontes and the Mitfords. Her favourite pair of spectacles are the Janet (Frames) and she has just designed the Eleanor (Catton).

Art was the other obvious touchstone. Kate's collection Art Groupie referenced the Surrealists; she used Christo-like wrapping as a key theme in Take a Hike, Vanessa Beecroft's nude works were the backbone of her Nouveau Match range and When Jackson met Claire was a mash-up of Jackson Pollock's splatter prints and one of Kate's all-time fashion idols, American modernist Claire McCardell. McCardell can be seen in all of Sylvester's work - part of a triumvirate of influence alongside Coco Chanel and Miuccia Prada.

It was an art collection, The Kiss, based around the painting by Gustav Klimt, that marked the first time Sylvester worked with stylist Karen Inderbitzen-Waller. That was in 2001 and they have collaborated ever since - making for one of the longest-standing relationships in the business.

The third key influence in the axis has always been music. Kate cannot design without bFM blaring out of her office in Central Rd in Kingsland. And even though she and Wayne might have VIP box passes to Laneways, they are always the first to run up front to the mosh pit when a band they love comes on stage.

Their collections have frequently been defined by their soundtracks such as the AK79 punk anthems that Wayne chose for the runway show for Royally Screwed, or Into My Arms by Nick Cave. It was Cave who prompted Kate to design the Stop Your Sobbing collection in the first place and it remains one of her most beautiful and heart-rending collections and a powerful, spellbinding runway show.

The music Wayne chooses for their shows is always unexpected and yet perfect: The raw grunt of Jesus Built My Hotrod for the Wolf show changed the mood from Little Red Riding Hood fairy-tale to Westie as hell in a heartbeat. They have a rotation of favourites - Iggy Pop, Bowie, The Jesus and Mary Chain, but will always throw in new loves that work for the mood of the moment - Arcade Fire, The Horrors, Brian Jonestown Massacre.

The mood at their shows is always a party - and this is partly because they love parties and seem to have calculated the crucial combination of the right people and factors to create good ones.

A ticket to a Kate Sylvester after-party has always been the most covetable one on the fashion week schedule and so many times when I was a fashion journalist I was angry and bitter about having to go home and write a review while all my friends stayed back with Wayne and Kate to drink and talk for hours without me.

Back in the early 2000s, when three children arrived in quick succession - twins Tom and Ike, and then Cosmo - everyone assumed the boys would inspire a childrenswear range.

Fed up with this, Kate did the opposite and designed a collection taking inspiration from childrenswear instead. Le Petit Garcon was revealed in an off-schedule fashion week show that is one of my favourites of all time. It was a small gathering at Sheinkin cafe where the models, dressed like schoolboys, walked up to the chalkboard and wrote out "lines" while we drank champagne.

I can't list all of my favourite shows; 21 years means 42 collections.

It is a long time to be in the business and people who don't know better will tell you that fashion is defined by youth. But the real power in fashion lies in age - think Karl Lagerfeld and Miuccia Prada. The best designers grow older but their label is eternal. Twenty-one? Just a whippersnapper.

Happy birthday Kate Sylvester. And may there be many, many more.

• Take a look back on some favourite Kate Sylvester collections and moments below.