Prince Harry is usually the one ribbing his older brother about hair loss whenever he gets the opportunity.
But Kate could not help teasing William about his disappearing follicles when the royal couple visited the prestigious Sydney Royal Easter Show.
As they toured elaborate displays of produce grouped by Australian regions, one exhibitor, Lyn Crejan, 67, talked about the wealth of fruit and vegetables in a colourful design behind her.
When she showed them a tuft of alpaca wool - which was a similar shade of brown to the Duke's hair - the Duchess, who wore a white Zimmermann dress, joked about her husband using it as a wig.
Ms Crejan, a farmer from the settlement of Glenn Innes in New South Wales, said: "The Prince was interested in the alpaca and as I showed it to them the Princess said he should put it on his head.
"She said, 'you need it more than me', and pointed to his head and he laughed."
William's hair loss is similar to that of his father the Prince of Wales and uncle the Earl of Wessex, and in recent months even Harry has begun to develop a bald patch.
Billed as an event that "brings the country to the city" the 14-day Sydney Royal Easter Show attracts more than 900,000 visitors every year, and has been a fixture in the calendar of farmers, animal lovers, gardeners and the general public since it was first held in 1823.
Royalty has been associated with the show since Queen Victoria gave permission in 1890 for the event to use the royal prefix.
The Prince of Wales visited in 1981 and 10 years earlier the Duke of Edinburgh toured the stands.
Featured in the show are prize-winning livestock from beef cattle and merino sheep to goats and horses, while animals you would not expect to see in the farmyard, like lizards and alpacas, are also included.
More than 10,000 items of produce are on display and there are stands offering activities including chuck washing, pig patting and cow milking.
As the royal couple entered the Cox Pavillion where a sheep shearing display was under way, a ram with impeccable manners made them smile when he welcomed them with a bow.
William and Kate were left marvelling at the formal greeting from Fred, a six-year-old merino ram.
The well-trained sheep went down on one knee with a little help from his owner, sheep farmer Jim Murray.
The Royal couple later visited Bear Cottage children's hospice, where they met sick children and Kate gave her only speech of the tour. She said she wanted to create a "community of best practice" with East Anglia Children's Hospice in the UK, of which she is patron..