Former US president Barack Obama addressed an invite-only crowd at the Art Gallery of NSW after touching down in the Harbour City on Friday.

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No tickets were available, but some of the lucky invitees posted to social media to shed some light on the subject matter of Mr Obama's speech.

City of Sydney councillor Christine Forster tweeted a few points from the address which touched on the importance of young people and women.

"When I say that companies need women on boards it's not for charity — those companies run better: President Barack Obama," Ms Forster tweeted.

"Young people drive change because they're not vested in the way things have been, they can imagine how things should be. The problem is they don't have any faith in existing institutions: President #Obama," she also posted.

Barrack Obama is due to give a speech at the Art Gallery of NSW tonight. ( ABC News )

It is not known exactly who scored an invite, but a spokeswoman described them as "businesspeople".

Two more guests who have since revealed their attendance through social media are celebrity chef Matt Moran and City of Sydney councillor Linda Scott.

Pre-speech Opera House visit

Making the most of his Friday afternoon in Sydney, Mr Obama took a tour of the Sydney Opera House where he was asked by a crowd what he made of Sydney.

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"Gorgeous, beautiful, love it," he said.

The 44th president also met with Prime Minister Malcolm Turnbull, who tweeted a photo of the pair captioned "welcome back".

Mr Obama was invited to the region by the New Zealand United States Council, whose members include businesspeople and academics keen to bolster economic links between the two countries.

The former president has just completed a three-day visit to New Zealand.

On Thursday night, actor Sam Neill hosted a discussion with Mr Obama at an event in Auckland.

Mr Obama also met New Zealand Prime Minister Jacinda Ardern and former leader John Key.

His regional visit will also take in Singapore and Japan.