The billionaire was joined by a mystery brunette as the pair marvelled at the sights (Picture: SWNS)

Bill Gates was keeping a low profile as he enjoyed a day outside the Sydney Opera House.

The billionaire was joined by a mystery brunette as the pair strolled by the iconic landmark.

They visited the Opera House on Monday and, according to an onlooker, went about the visit largely unnoticed.

Mr Gates, 67, the co-founder of Microsoft, was ranked fourth on last year’s Forbes rich list.

He sported a casual baseball cap, blue polo shirt and trainers in Sydney.

Unless hidden in plain sight, any security teams were nowhere to be seen.

The onlooker who spotted Mr Gates later said: ‘Nobody else noticed who Bill was or approached him. He was obviously trying to keep a low profile.

Bill Gates sported a casual outfit and was seen standing by the mystery woman (Picture: SWNS)

The billionaire is in Australia on business (Picture: SWNS)

‘No one would have had any clue that one of the world’s richest and most powerful men was in their vicinity.’

Mr Gates is in Australia on business and met with the country’s prime minister earlier this week.

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The billionaire sported a casual outfit for the occasion.