The royal couple, who are currently visiting Mary's native country, witnessed the devastation caused by fires in the town of Winmalee, 80km from Sydney.
Dressed down in jeans and a white lace t-shirt Mary walked around the fire-ravaged town and spoke to firefighters who had eventually managed to quell the blaze.
Frederick was seen speaking to locals, volunteer firefighters and New South Wales' Rural Fire Service Commissioner Shane Fitzsimmons.
Well-wishers, particularly the children, were thrilled to catch a glimpse of the princess who grew up in Tasmania, one of them giving Mary a pink rose.
Later in the day Mary and Frederik stepped out at a concert celebrating Sydney Opera House's 40th birthday.
Mary chose a sparkling cream and silver dress, which she wore under a cream coat.
The mum-of-four wore her brunette hair in a chignon and accessorised with gold earrings and nude studded heels.
Mary and Frederik's attendance at the concert was particularly apt as the iconic opera house was designed by Danish architect Jørn Utzon.