William and Kate plant a tree in Canberra

Young Canberra twins have helped the Duke and Duchess of Cambridge plant a tree at the National Arboretum.

William and Kate kicked off their Thursday tour of the capital by becoming the first members of the royal family to add to the arboretum's collection.

Local six-year-olds Oliver and Sebastian Lye were on hand to help the royal couple plant the English oak tree near the arboretum's Pod Playground.

Sebastian Lye has Down Syndrome, and is both vision and hearing impaired.

The oak was chosen because it is a symbol of England, representing strength and endurance. The arboretum was established following the devastating 2003 Canberra bushfires.

William and Kate both shovelled soil on to the tree's base before the boys watered it with red watering cans.

The couple then mingled with 50 lucky Canberrans who won a ballot to be a part of the event, even playing with children in the playground.

Prime Minister Tony Abbott will later welcome the couple to Parliament House for an official reception where William is expected to make his second and final speech in Australia.

Monarchists are hoping to hand over a gift of a half boulder Opal to the duchess during the event.

Later, art will come to life at the portrait gallery where William and Kate will meet several of the artworks' subjects.

They will then head back to their home-away-from-home, the governor-general's residence in Yarralumla, for another reception.