Perth commuters are facing a tough trip to work Thursday morning due to a combination of heavy fog and a bus drivers strike.

The Bureau of Meteorology said the fog was caused a weak low pressure system south of Rottnest pushing a shallow layer of moisture over the coast.

Visibility is expected to be low on Perth roads until about 8am.

Main Roads has warned the fog is particularly strong on Mitchell Freeway between Hester and Whitfords Avenues and Tonkin Highway in Redcliffe.

Extra traffic is expected on the roads as services across Perth as bus drivers from Transdev WA go on strike from 6am to 10am.

The company provides CAT bus services in Perth, Fremantle and Joondalup as well as general Transperth services across the city.

Transport Workers Union national president Tim Dawson said on said on Tuesday dozens of routes in these areas would be affected, including the free CAT bus service in the city.

“Our bus drivers find no pleasure in inconveniencing the community and travelling public, but our bus drivers have been patient for too long,” he said.

The company’s Perth managing director Ben Smith urged public transport users to make alternative travel plans where possible.

“We’ll be getting as many buses on the road as we can, with a focus on ensuring school services run as close to normal as possible,” he said.