The Duke and Duchess of Sussex are expecting their first child, due in the spring of 2019, the couple's office Kensington Palace announced on Monday.

The baby will be seventh in line to the British throne.

The Duchess is believed to be about 12 weeks pregnant.

'Their Royal Highnesses have appreciated all of the support they have received from people around the world since their wedding in May and are delighted to be able to share this happy news with the public,' the palace said in a statement.

The news emerged shortly after the royal couple landed in Australia for their first overseas tour, a busy trip which will also take in New Zealand and the South Pacific islands of Tonga and Fiji.

Harry, the son of Prince Charles and his late first wife Princess Diana, and Meghan, former star of TV show Suits, married in a glittering ceremony in St George's Chapel, Windsor Castle five months ago that attracted huge interest worldwide.

The couple returned to the castle for the marriage of Harry's cousin Princess Eugenie on Friday which was attended by all the senior members of the British royal family. Media said Harry and Meghan broke the news of the pregnancy to other members of the royal family at the wedding.

The couple have made no secret of their wish to have a family, with Meghan saying in an interview on their engagement that becoming a mother was on her 'bucket list'.

Additional reporting by Reuters.