"We have been very anxious to have you here so we could adopt you as well," he told the royal couple at a reception at Admiralty House on Tuesday afternoon.

Meghan changed into a dark olive shirt dress with a full skirt for the event after a busy morning at Taronga Zoo and meeting the public at the Opera House.

The reception is being attended by Australians from a broad range of sectors, including charity and community, arts and culture, and entertainment and sport.

Sir Peter congratulated the Duke and Duchess of Sussex on the news they were expecting their first baby.

He also praised the focus of the visit - the Invictus Games for war veterans - that has been promote by the prince, who served in the British army for a decade including two tours of Afghanistan.

"It was born from your genuine empathy with your fellow veterans. And it struck a chord immediately," Sir Peter said.

Prince Harry also addressed the reception, joking about the catering.

"We are delighted to be here, and really impressed to see you serving beer and tea at an afternoon reception, in true Aussie style," he said.

"Thank you for the incredibly warm welcome and the chance to meet so many Aussies from all walks of life.

"And we also genuinely couldn't think of a better place to announce the upcoming baby, be it a boy or a girl."

Australian Associated Press