A search is underway for two men overdue from a diving trip in Southern Otago.

The pair - aged 23 and 51 - were diving recreationally for paua in Purakaunui Bay.

A police spokeswoman told the Otago Daily Times that the men are not related.

The alarm was raised about 7.30pm yesterday, when police were told the two Balclutha men had not returned.

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Authorities said this afternoon search teams were called out immediately and worked until midnight to try and find the men; but were unsuccessful.

Search for the divers resumed at this morning at 6 o'clock.

"A land and marine search is underway - focused on the area between Irihuka (also known as Long Point) and Nugget Point,'' police said.

"Several boats and [inflatable rescue boats] are out on the water and a LandSAR team is conducting a coastal search.''

The Police National Dive Squad has also been called in to help in the search and a helicopter and fixed-wing aircraft are now also involved.