Lorde says she racked up the air miles to make a surprise appearance at a Taylor Swift concert because the pair are "best buds".

The Kiwi songstress flew 19 hours to join Swift as a special guest at her Washington DC show, where the pair performed Lorde's biggest hit song Royals together.

Swift had promised the crowd a "really good surprise" if they "maintained their awesomeness".

"Hint number one: this artist flew 19 hours from her hometown to be here with you," she told her screaming fans.

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Lorde was then introduced by Swift as "one of my best friends in the entire world".

They sang Royals together, harmonising together on lines like, "We'll enjoy being queens".

After the show, Swift took to Twitter to call Lorde a "wondrous angel".

Lorde returned the compliment, calling Swift "the leader of a great and beautiful galaxy" and explaining why she'd flown so far just to perform one song with her.

Lorde's appearance lit up Twitter, with one fan saying she was so sad she missed it she was in tears.

The friendship between Lorde and Taylor Swift dates back to 2013. Initially, Lorde criticised Swift as being a poor role model for young girls, describing her as "so flawless, and so unattainable". Lorde later apologised for her comments and clarified them further on Tumblr.

The two singers later put any rumours of a feud to rest when they were spotted dining together at US chain restaurant Shake Shack. They've been close friends ever since.

Lorde recently praised Swift for bringing her into "this amazing world of supportive female friendship" in an interview in Dazed magazine with Girls star Lena Dunham.

- nzherald.co.nz