* Crowds greet royal couple in Australia to celebrate baby news
* Kensington Palace says Harry, Meghan expecting child in spring
* Couple visiting Australia, New Zealand, Fiji and Tonga
* Markle's outfit a big hit (Updates with detail on dress, designer's website crashes)
By Paulina Duran
SYDNEY, Oct 16 (Reuters) - Crowds thronged the Sydney Opera House and the shores of the city's sparkling harbour on Tuesday to catch a glimpse of Prince Harry and his wife, Meghan, on their first public appearance in Australia since the announcement they were expecting a child.
The royal couple began their official tour of Australia with a ferry ride across the harbour to greet cheering crowds packed in front of the Opera House, which was opened by Harry's grandmother, Queen Elizabeth, 45 years ago.
"We're both absolutely delighted to be here. We genuinely couldn't think of a better place to announce the upcoming baby, be it a boy or a girl," Harry told a reception at Admiralty House, the official residence of the governor-general.
Kensington Palace said on Monday the Duke and Duchess of Sussex were expecting their first baby in the northern spring. The child will be seventh in line to the British throne and the queen's eighth great-grandchild.
Thousands of people gathered on the harbour and at the Opera House, where the couple shook hands and embraced members of the public.
"I want to see the royal couple. They are young, a breath of fresh air for the royal family and Australians love them," said Marvin Lester, who has attended every royal visit to Australia since 1954.
"There's a lot of feeling for a republic here in Australia but I admire having a royal family," she said.
Australia, a constitutional monarchy whose head of state is the British monarch, voted in 1999 against becoming a republic.
Support for a republic is hovering around 50 percent according to a January poll, a level largely unchanged in recent years, and there is little appetite to put the issue back on the agenda during Queen Elizabeth's reign.
Governor-General Sir Peter Cosgrove, the queen's representative in Australia, presented the couple with their first baby gifts, a stuffed kangaroo and a tiny pair of fleece-lined Ugg boots.
The visit to Australia is their first overseas tour since Harry, 34, married the former Hollywood actress in May. The trip will also take the couple to New Zealand and the South Pacific islands of Tonga and Fiji.
The couple met some Australian wildlife during a visit to Taronga Zoo in Sydney, where zookeepers said the 3,500 animals there were "excited to meet them".
Excitement over news of the baby was almost matched by a frenzy over Meghan's outfit.
Named 2018's best dressed woman by People magazine, Meghan wore a white cocktail dress by Australian designer Karen Gee underneath a beige trench coat that revealed just a hint of a baby bump.
The A$1,800 dress, dubbed "Blessed Dress", was an instant hit, with demand crashing the designer's website a number of times throughout the day.
After watching an indigenous-themed show by the Bangarra Dance Company at the Opera House, the couple met prominent figures from sports, business, the arts and charities.
It is the second tour of Australia by the younger generation of royals after Prince William and his wife, Kate, and their eldest son, George, visited in 2014.
The trip by Harry and Meghan coincides with the Invictus Games to be held in Sydney from Oct. 20-27. The games founded by Harry are an international paralympic-style event for military personnel wounded in action. (Reporting by Paulina Duran; Additional reporting by Tom Westbrook; Editing by Paul Tait, Robert Birsel)
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