Queen Mary's wedding dressmaker on 'rather terrifying' process of making Aussie royal's dress for the big day

When Australian-born Mary Donaldson walked down the aisle to marry her prince, Crown Prince Frederik in 2004, she had a little extra good luck on her side.

The Danish designer and dressmaker Birgit Hallstein who worked on her wedding gown, tells 9Honey she used an old tradition for the bride's dress.

Finishing it on the very day of the wedding is something that she thinks may have been suggested by Mary's new mother-in-law, Queen Margrethe.

READ MORE: Mary and Frederik mark 20th wedding anniversary in Norway

But regardless of who suggested it, as the couple celebrate their 20th wedding anniversary on Tuesday - and their first as the newly minted King and Queen of Denmark - it appears to have done the trick.

Hallstein admits the old tradition, which involves doing the final loops and stitches the morning of the wedding, also helped her.

"[It] was a thing I did because it's a tradition in some families, but honestly I don't remember if Queen Margrethe would have mentioned such a thing," Hallstein tells 9Honey.

"Maybe, because she knows a lot of those things... I am sure we talked about it, but anyway it was practical to gather the dress on the day because its big."

The process for Hallstein began in November 2003 and involved working with Danish designer Uffe Frank and using pearl white heavy silk duchesse fabric and antique lace to bring the creation to life.

READ MORE: Looking back at Queen Mary's 2004 wedding outfit 20 years on

"It was rather terrifying," she admits with a laugh of making the Crown Princess-to-be's wedding dress.

"It's a bit like an exam, just a really big one, because if you fail everyone will see."

Hallstein worked in a space at Amalienborg Palace to deliver the gown as well as bridesmaids, page boys and flower girls' outfits.

"The sewing took hundreds of hours, starting in January 2004 and ending right before the wedding," she recalls to 9Honey.

There were approximately six fittings between January and May 14, 2004.

These were mostly held with Mary, her stylist and the dressmaker but Hallstein says "sometimes the Queen attended".

There was also the added pressure of royal traditions and protocols around the use of the antique veil and the lace attachment to the petticoat, which was from Queen Margrethe's private collection.

"There are rules to follow, [you're] not allowed to cut in it and only skilled repairs [are allowed]," Hallstein tells 9Honey.

"I had to hide around two or three meters of the antique lace in between layers of organza inside the dress to make sure it was not damaged by high heels, chairs, cars and carriages during the day."

In a social media post on Tuesday to mark 20 years since the world got their look at the classic gown, the dressmaker explained to fans that "the wedding gown consists of three parts".

There's "a big tulle petticoat, edged with almost 60 yards of Chantilly lace, on this a big light blue silk bow were placed, to make sure the first born would be a son," she said.

"Covering the tulle there was a silk duchesse petticoat with a beautiful antique lace on top.

"The bodice with all the draped panels were looped to the two petticoats. The [six metre] long train was attached to the dress with hooks."

And just like the good luck tradition for a long marriage, it looks like the blue silk bow worked too with their son, now Crown Prince Christian, born almost 18 months later, in October 2005.

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