The more than two week official visit will take in multiple stops in Australia, Fiji, Tonga and New Zealand -- all parts of the Commonwealth of states formerly colonised by Britain.

Queen Elizabeth II is technically still the head of state in Australia, although for practical purposes the country is independent and there is significant support for becoming a republic.

The royals are likely to pet koalas, watch soldiers perform the haka and attend the Invictus Games, a multi-sport tournament for injured military veterans that opens on Saturday 20 October.

The whole visit will be keenly watched by the British media.

Meghan, a photogenic US actress, married the grandson of Queen Elizabeth II in May, becoming the new royal of media obsession in Britain and beyond.

The 37-year-old made waves recently by closing her own car door.

The couple will be accompanied by a staff of 10 and a substantially larger swarm of reporters from all the British papers and TV stations.

The official visit begins on Tuesday in Sydney, where they will be welcomed by Australia's Governor-General Peter Cosgrove and his wife Lynne.

The royals will meet two koalas that same afternoon.

After sailing Sydney Harbour and visiting the famous Bondi Beach, attending functions at Prince Harry's Invictus Games competitions for wounded soldiers and meeting Australian Prime Minister Scott Morrison, they jet off to Fiji on October 23.

Meghan will get a chance to speak at a UN women's empowerment in marketplaces event and meet some female vendors in the project in the Fiji capital Suva.

They will meet the local royals in both Fiji and Tonga, which are members of the Commonwealth.

The final leg coincides with the 125th anniversary of women's suffrage in New Zealand -- the first country to give women the right to vote.

Meghan is expected to speak at a suffrage anniversary celebration in Wellington on October 28.

The couple will have a chance to watch members of the New Zealand defence force perform the haka -- the traditional warrior dance popularised globally by the nation's rugby team -- at a governor's reception earlier that afternoon.

The trip officially ends in New Zealand on October 31.

Meghan has previously visited New Zealand as a tourist in 2014. Prince Harry has been to all four countries, but on his own.

The couple's first trip abroad saw them dash off to Dublin shortly after their wedding.