Commuters were advised to find alternative travel arrangements after 115 bus drivers called in sick in Sydney's north west on Wednesday, causing a number of service cancellations.

People traveling from the CBD to the north west faced the longest delays, with Hillsbus urging customers to leave work early if possible.

"We are working to operate as many services as possible, however, there will be some cancellations and delays," a Hillsbus spokesman said.

"Hillsbus sincerely regrets the disruption and inconvenience caused to our customers."