North Shore model-actress Nicola Simpson has modelled for Christian Dior, Elle and Adidas and is in the midst of filming the next series of American TV hit Spartacus.

But yesterday her toga garb was swapped for a replica of Kate Middleton's wedding gown, whipped up overnight by Kiwi couturist Jane Yeh.

Yeh worked through the night to create the dress, identical to the Sarah Burton number that had been revealed only 15 hours earlier.

What Middleton was to wear on her big day had been a closely guarded secret, but Yeh said she was able to make educated guesses and stock up on likely fabric choices.

"[Kate] has darker skin so I knew pure white wouldn't be her colour," Yeh said.

The designer, who had only a one-hour nap during the night, was relieved Kate had not emerged in a 25m train - but said the replica still had more than 40m of ivory silk satin and tulle.

$250-a-metre French lace was used on the bodice, veil and skirting detail.

Simpson said the dress was heavy but comfortable.

"I wouldn't want to sleep in it, but the corset is cushioned on the inside so it is quite comfortable," she said.

Yeh's replica was unveiled at Westfield St Lukes and will travel the country, appearing in 18 Farmers stores over the next 22 days.

The dress will be accompanied by full-size replicas of a range of crown jewels, including a copy of Middleton's engagement ring.