The elegant royal, who hails from Tasmania, joined in with a rendition of Advance Australia Fair - the national anthem of her homeland – during a trip to Sydney with Prince Frederik.
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Mary and her husband had dropped in on the city's Five Dock primary school, where they received a warm welcome from children and teachers.
The 41-year-old and her husband seem to be in their element as they make a return to the city where they fell in love after meeting at the 2000 Sydney Olympics.
Stunning in a black A-line tailored black dress with matching pumps and clutch and her brunette locks swept into a chic up'do, Mary went about the day's activities, her smile never leaving her face for a moment.
The style-conscious royal opened a fashion store featuring products from Copenhagen-based luxury brand Ole Lynggaard.
On the second day of their tour she also joined Frederick at the Royal Opera House, where the crown prince spoke at an event to launch MADE (Multidisciplinary Australian Danish Exchange).
"The crown princess and I are as always delighted to be back in Australia and particularly in Sydney," he said.
The couple met dignitaries including Lord Mayor Clover Moore, who revealed the subject of an interesting chat she shared with the Danish royal mum when discussing her campaign to get more Sydney residents ditching cars in favour of cycling .
She posted on Twitter: "Princess Mary told me she takes her kids to school on a bike!"
Frederik and Mary will be in Sydney for five days to attend events to celebrate the 40th anniversary of the Sydney Opera House and the Danish architect who designed the landmark, Jorn Utzen.