Lorde wore a gown from French fashion house Balenciaga for the Grammys ceremony.
The floor-length black dress - thought to be made from Japanese silk - is from the designer's pre-autumn collection.
Lorde wore her usual deep purple lipstick after a makeover by MAC Cosmetics senior artist Amber Dreadon, who told the Los Angeles Times she wanted to "keep the look fresh and radiant - modern, a bit boyish and darkly cool with a focus on the lips".
Lorde's trademark brunette curls were straightened.
True to her indie style, she did not show up on the red carpet, where celebrities are typically asked which designer they are wearing, leaving many speculating on the Twittersphere.
For her performance of Royals, the Kiwi star wore a white sleeveless shirt by Italian fashion house Prada and loose black pants from French luxury goods brand Celine.
She had social media talking with her unusual choice of nail art.
Her simple monochrome outfit was teamed with black-smudged fingertips which looked as though they had been dipped in ink.
The odd fashion choice quickly prompted its own Twitter account as well as being likened to a symptom of the Black Plague by one US entertainment news website.
Lorde's fingernails - using the Twitter handle @lordefingernail - tweeted updates, with one saying "I would like to thank my thumb and pinky, without them, these black fingertips would not be possible".
That was followed by: "@lordemusic can split an atom ... with her fingernails. #justthetip."
And: "The #grammy party starts when Lorde's fingernails walk in. #partylikearoyal."
The look did not go down well with entertainment reporters, MTV describing them as "Black Death".
"During her performance of her hit song Royals, the 17-year-old singer sported black-tipped fingers, which are one of the symptoms of the European pandemic known as the Black Death, or the Black Plague," it reported.
Elsewhere, the Huffington Post asked its readers whether the fashion statement was "freakin' fantastic" or "totally terrifying".
It was slightly more positive though, describing the pop star's look as "bad ass" and "creative and a bit creepy".
Lorde had ditched the look when she took to the stage again to collect her award for best pop solo performance.
- additional reporting Patrice Dougan of APNZ
High school to hang out huge banner
Lorde's high school will celebrate her achievements by hanging a huge congratulatory banner on the main building for students as they return from their holidays today.
Takapuna Grammar principal Simon Lamb said: "Naturally, Takapuna Grammar school joins the local community in its support and admiration of our local talent, Ella Yelich-O'Connor. We are very proud of her achievements as recognised by the double ... win.
"We remain in close contact with Ella and her family.
"I am very proud of her, her music and her achievements.''
Takapuna Grammar head boy Harry Elworthy told TVNZ's Breakfast that he could not believe Lorde's success at the awards.
"It's really incredible to have someone who's so famous be so real at the same time. I couldn't believe it, really.''
He agreed she was always pushing boundaries.
"She's always been kind of an eccentric girl, she's always liked to be a little bit different. But definitely people follow what she does.''
He said Lorde would likely be happy to put on a private performance for her schoolmates, and bringing along her Grammy trophies was "almost an obligation''.
Takapuna Grammar deputy head girl Sophie Wynn said Lorde would likely be "a little bit embarrassed" because of the banner.
"Everyone driving past can see, just congratulations - the school's very, very proud of her,'' she told Breakfast.
Ms Wynn, who has been at school with Lorde since primary school, said the star had always been involved in drama and performing.
"She's always put herself out there as one of those people who could do something like this, and she has, and it's amazing,'' she said.
"I don't think she's changed too much at all. She's still very, very friendly, willing to talk to anyone really. She's still very down to earth, really nice.''
Ms Wynn agreed Lorde would be a global icon for teenagers.
Meanwhile, Lorde's former music teacher at Belmont Intermediate School, Jenny Bezuidenhout, watched the awards with her fingers crossed yesterday.
She cast the teenager as the lead in a musical she wrote called The Case of Greed, and also started the school's "Idol'' competition which Lorde won when she was 12.
"So I'm sitting here saying to my husband 'Well, she sang my music before she sang her music'.".
Mrs Bezuidenhout said she ran into Lorde's mother at a shopping centre before Christmas.
"She said, 'What do you buy a girl that's got everything?' I just said to her then, 'Ella is a gifted individual and she has the potential to really influence her generation through her art'.
"She inspires me ... she's really in touch with herself and how she feels about things and doesn't rely on the outside world to dictate how she should feel or think. And I really admire that in her because she's really young."
Her family had really nurtured her talent and had to take a lot of the credit, the teacher said.
"When she came to school last year, she's just the same as she ever was. She's just a lovely individual with an amazing presence and is very, very bright. She's got it all."