Container ships at Fremantle Port were shrouded in the heavy fog
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Container ships at Fremantle Port were shrouded in the heavy fog.
A rare sea fog has moved over Western Australia's south-west coast, prompting a warning for people on the water to take extra care.

Boaters have been urged to be aware of the conditions as the thick fog moves past Rottnest Island and south off Perth's coast.

"We'll definitely see some fairly reduced visibility over the coastal waters during today, which is potentially quite hazardous for boats out there," Bureau of Meteorology (BOM) duty forecaster Max Strack said.

The fog emerged off the coast of Geraldton last night and gradually developed south.

Ms Strack said the fog was caused by a low pressure trough drawing warm, moist air from the north.

Sea fog is only seen off WA's coast a couple of times a year, with the current conditions likely to last into the afternoon.

"It is a moderately unusual event to have such a broad event of sea fog off the coast of WA," Ms Strack said.

The BOM said fog formed the same way as clouds and happened when the air temperature cooled to the dew-point temperature, which is when condensation occurs.

Fremantle Port said while the fog had been pronounced at the port, it had not disrupted its operations in any way.