Hundreds of bus drivers are expected to walk off the job in Victoria next week, disrupting dozens of routes in Melbourne and across regional centres.

Nearly 600 CDC Victoria drivers plan to stop work from Monday's last bus until Wednesday morning after rejecting a 2.5 per cent pay increase, according to the Transport Workers Union.

Its national vice-president, John Berger, says CDC's "sub-standard and insulting" offer is lower than previous average increases dating back to at least 2006.

Drivers are instead demanding a four per cent pay rise.

"Imagine being told by your boss that you are worth less this year than for the past 12 years - regardless of the impact this will have on your life and the increasing cost of living," Mr Berger said in a statement on Saturday.

"These drivers work long hours from early morning to the dead of night in performing an important community service."

CDC says the strike will affect 49 routes in Melbourne's east and west, and 25 services in Ballarat and Geelong.

Chief executive Nicholas Yap said the company's pay offer "demonstrates our support for (drivers)" and he apologised for the disruption to passengers.

PTV chief executive Jeroen Weimar urged the union and company to come to an agreement quickly.

"This is a disappointing development, which has the potential to cause real inconvenience to many of our bus passengers, particularly as people travel during school and university holidays," he said.