Princess Mary re-wears her favourite budget $99 H&M dress

Denmark's Crown Princess Mary is officially back from holidays but continues to show off her summer glow.

Mary, 47, has returned to a diary full of royal engagements and carried out two in Copenhagen overnight.

The first was for the Crown Princess's charitable organisation The Mary Foundation and focused on the release of a new report on social exclusion.

The Australian-born royal has long championed against loneliness and the need to include those members of society at risk of isolation.

Mary spoke at the report's launch, highlighting one of the key findings: that vulnerable people dream of a meaningful everyday life and seek employment but, due to labor market shortages, finding a job can be a challenge.

The Crown Princess then moved on to her second and final event of the day where she spoke to school children, in grades five and six, about the importance of sustainability.

In keeping with her message, Mary wore a dress from H&M's Conscious Collection, which uses a mix of recycled and organic materials.

The dress is made of organic silk and retailed for $199 before being reduced to $99. Unfortunately, it's now out of stock.

Princess Mary first wore the dress in May at the launch of the Copenhagen Fashion Summit, which is the world's largest event on sustainable fashion.

Mary, who is patron of the summit, said at the time: "Profit cannot continue to be the primary measure of success.

"Sustainable transformation must be a business imperative and a core priority".

The dress was also worn by Sweden's Crown Princess Victoria twice this year. Like Mary, Victoria has worn pieces from the environmentally-friendly H&M line for official events.

Mary frequently re-wears items that have been in her royal wardrobe for years, proving that if a princess - and future queen - can recycle, then so can we!