Video footage shows a person — appearing to only be wearing boxer shorts — emerge from the water as fish dart about.
He rises to the surface for air before diving back down and heading towards the glass, the video, which was posted on social media, shows.
New South Wales Police Force told The Independent: “Officers from Blacktown Police Area Command are investigating reports of a man trespassing at a zoo in western Sydney.”
The force said they have been told the man went into the zoo’s aquarium section around 3pm local time on Monday and swam in the tank.
“The zoo was open at the time,” they said.
Jake Burgess, managing director at Sydney Zoo, told 7News Sydney: “We are completely appalled that people would behave in this way.
“It not only jeopardises their own safety but that of the welfare of our animals as well.”
Footage of the incident shows fish swimming around while someone behind the camera says: “What a tank.”
The animals start darting about suddenly, as a man appears in the water.
Police have said inquiries are ongoing.
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