Who are Denmark's royal power couple, about to be throned king and queen?

Denmark's soon-to-be king and queen have sought to balance their royal jobs with a modern family life and an engagement in progressive causes, helping keep the monarchy relevant and popular in the Nordic country.

Crown Prince Frederik, 55, will take the throne on Sunday, following his mother Queen Margrethe II's surprise announcement on live TV on New Year's Eve that she would abdicate in favour of her oldest son.

He will become King Frederik X and by his side will be Queen Mary, 51, an Australian-born former marketing and advertising director.

The couple will take the throne at a time of massive public support and enthusiasm for the monarchy in Denmark, recent opinion polls show.

Queen Margrethe's tactful and traditional manners and interest in history and art brought her strong support.

Frederik and Mary have, on the other hand, used their fame and goodwill like modern influencers to modernize the monarchy.

But their fight for environmental issues, sustainability, public health and rights of girls and women globally have also gone down well in Denmark.

"In many ways, they are a modern and equal power couple," said Jakob Steen Olsen, a commentator on the Danish monarchy. "You can say they are more activist and use their status to make the world a slightly better place."

The couple has been spotted on the streets of Copenhagen taking their kids to school on a cargo bicycle - a popular means of transport for climate-conscious Danes.

Contrary to his mother, who has been a heavy smoker throughout most of her life, Frederik is an advocate of active and healthy lifestyles. He organises a hugely popular running event, Royal Run, each year where he and his family join thousands of Danes running through the streets of Copenhagen and other towns across the country.

"It's their way of modernizing the monarchy and keeping it relevant for future generations," said Olsen.

The two were introduced by common friends at a party in Sydney during the 2000 Olympic Games. Mary later recalled in an interview that she did not know he was a Danish royal the first time they met.

"It was perhaps half an hour or so later that someone came up to me and said: Do you know who these people are?" she said.

The couple married four years later at a grand ceremony in Copenhagen. They have four children, Prince Christian, 18, who is heir to the throne, Princess Isabelle, 16, and twins Princess Josephine and Prince Vincent, both 13.

In his youth, Frederik was a rebellious teen who often seemed uncomfortable in his role as heir. After graduating from university, he became known as an adventurer and for his interest in sports.

He is a Frogman in the Royal Danish Navy's special force, whose training is one of the world's toughest; has run six marathons; and participated in a four-month, 2,795-km (1,737 mile) dogsled expedition in northern Greenland, a sovereign territory of the Kingdom of Denmark.

In 2018, he took the stage at a music festival with one of Denmark's most popular rock bands.

"You can talk to everyone, and you believe that we can all contribute and learn from one another," Mary said in a speech to her husband on his 50th birthday.

Princess Mary, who was born Mary Elizabeth Donaldson in Hobart, Tasmania, is possibly even more popular than her husband. She was admired for mastering the Danish language just a few months after arriving in Denmark.

She said wanted to bring "a little Aussie twist" to her role as princess. She has worked as patron to a number of organizations promoting issues close to her heart, including children's and women's rights and sustainability in fashion.

The most recent survey done after the queen announced she would abdicate indicated that 82% of Danes expect Frederik to do well or very well in his new role, while 86% said the same about Mary.