Kensington Palace has confirmed there won't be a change of itinerary to Harry and Meghan's tour despite warnings from health authorities to avoid the Pacific countries while pregnant.

The palace said the couple had sought medical advice and decided to continue.

Carried by mosquitoes in warm climates, the Zika virus has been linked to birth defects and complications if pregnant mothers become infected.

Tonga was in March listed by the World Health Organisation as a country where there was still on-going transmissions, while Fiji last had a confirmed infection in 2017.

However, according to the International Association for Medical Assistance to Travellers, the short time the couple will be spending in both countries reduces the risk of transmission.

The public may also see the Duchess wearing plenty of light colours and clothing with long sleeves if she follows WHO advice about avoiding mosquito bites.

Kensington Palace announced Meghan's pregnancy on Monday night Australian time, hours after the couple arrived in Sydney at the start of a 16-day tour that also includes New Zealand.

Their baby will be seventh in line to the throne, and the Queen's eighth great-grandchild.

WHAT'S ZIKA?

* A mosquito-borne virus linked to birth defects and complications.

* There is no known vaccine or treatment.

* Tonga was this year still reporting new cases, while Fiji hasn't confirmed any this year.

* Both the WHO and Britain's Foreign Office advise pregnant women against travelling to countries with infections.

Australian Associated Press