Kate was heard to say the outfit was making her "very hot" in the bright sunshine in Canberra.
She looked elegant as ever as she and Prince William planted a tree and chatted to schoolchildren during the visit.
The couple spoke to the youngsters about conservation projects during their 30-minute tour of the arboretum with its gardens, outdoor sculptures, amphitheatre, cafe, visitor centre and children's playground.
Then they went to the window to look at the stunning view - which included a sighting of Government House where George is staying.
The couple then planted an English Oak tree with the help of six-year-old twins Oliver and Sebastian Lye.
Sebastian has Down's Syndrome and also has a vision impairment and is hearing impaired - and wears glasses and hearing aids.
Sebastian recently received a little legend award from Gai Brodtmann for children achieving whilst dealing with adversity.
Their mother Moira Lye has had an ongoing battle with cancer over many years.
After Oliver and Sebastian watered the tree, Kate and William met selected school children who were involved with conservation projects and told Lillian Rowland, 15, from Alfred Deakin High School about their recycling habits.
She said: "We discussed what our school was doing towards being more sustainable.
"They were very interested.
"Kate said they were fanatical about recycling and when they have friends over if they put things in the wrong bin they will take it out and move it.
"Then the couple watched children play in the nearby play area before joining in, following them into little huts.
William spent several minutes in one hut and came out laughing: "Sorry, I got distracted."
Afterwards the couple met with some of the 50 members of the public who had won a ballot to be at the event.
New parents Daniel and Janice Spackman, both 36, were there with six week old Elsie.
Kate went over to them and said: "Congratulations, she's gorgeous."
Daniel said: "William asked is she sleeping well.
"I said 'I think she's a bit disappointed George isn't here today'.
"William said it's probably a good thing George didn't come.
"His hands and feet tend to fly everywhere with other babies."
Kate also spoke to Karen Vey, 39, who was with her son Harry, 10. They gave Kate children's book 'Where's the green sheep?'.
Karen said: "She said we were very lucky to have this beautiful weather, I
said I'm quite hot.
"She said she was very hot and I said 'you would be in that outfit."
Following the devastating 2003 bushfires in Canberra, the ACT Government dedicated an area of burnt-out pine forest west of the city as the site for
an innovative new national arboretum.
A national design competition was held for the new arboretum and in 2005 the winners were announced - Taylor Cullity Lethlean Landscape Architects and
Tonkin Zulaikha Greer Architects with their '100 forests and 100 gardens' joint proposal.
Their winning concept focused on the establishment of 100 forests of rare, threatened and symbolic trees from Australia and around the world.
The master plan for the site included a mosaic of gardens, outdoor sculptures, amphitheatre, cafe, visitor centre, bonsai and penjing pavilion, children's playground, pavilion and a spacious outdoor events space.
Designed by Taylor Cullity Lethlean Landscape Architects, Pod Playground uses the idea of seeds as the beginning of life in the forest.
Pod Playground houses giant acorns floating in the sky and enormous banksia
cones nestled on the ground. A large net fort, nest swings, music-making
instruments and a musical bridge offer creative learning experiences.
The playground is designed to create wonder, imagination and enchantment and to encourage activity and spontaneity in play.