A search is under way for a swimmer who disappeared at Maroubra Beach in Sydney.

Volunteers and vessels from Marine Rescue Botany Port Hacking and Marine Rescue Middle Harbour are searching for the man with water police.

The man, who police believe is in his 30s, failed to resurface after going into the water just before 4pm today.

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"Conditions are not favourable offshore this afternoon, with choppy seas and squally rain showers," Marine Rescue NSW Inspector Andrew Harding said.

"Volunteer crews on board Middle Harbour 30 and Botany Hacking 32 will continue searching this evening until instructed otherwise by NSW Police Marine Area Command."

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