Salvage crews will try to refloat a 50-foot trawler that became stranded off a beach in Sydney's south.

The fishing boat, Challenge, ran onto rocks near Cronulla beach after midnight on Monday.

Four Indonesian crew members tried to escape the choppy waves on a small boat before marine officers rescued them.

A salvage team have attached lines to the stern of the vessel which is expected to be towed about 4am (AEDT) on Wednesday, Sydney Ports marine service manager Phillip Holliday told AAP.

He said the boat is in good condition and no fuel has leaked so far.

"We're pretty confident it will be towed off. If things don't go as well we'll reassess from there," Captain Holliday said.

High tide is expected at 5am (AEDT) and it's expected the refloat will take several hours.

Captain Holliday said a salvage team, water police and volunteers from the State Emergency Service will stay at the scene overnight to maintain an exclusion zone.

Curious locals watched the spectacle from the Esplanade on Tuesday.

An investigation is under way into why the trawler ran aground.

"It looks to be a sound vessel, so how did it end up sitting there? It's not supposed to be there - so what went wrong?" Captain Holliday said.

The crew are being questioned.

 

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