A strike by rail workers will cause havoc on Wednesday, with trains on the South Coast running to a weekend timetable and buses ferrying commuters.
Members of the Rail Tram and Bus Union have decided they will not work on any privately built trains, which means only about 30 per cent of the services across the network will be running.
Transport for NSW chief operations officer Howard Collins said commuters should avoid peak hour travel.
RTBU secretary Alex Classens said the dispute was over senior managers not meeting with the union to negotiate fair wages and conditions.
"We don't want to have to take any action, but rail workers have absolutely no choice but to try to make management listen to them, and industrial action is the only tool they have," Mr Claassens said.
"Rail workers will again take every step to make sure essential workers can still get by while industrial action takes place this month, but we urge commuters to plan their travel carefully and leave extra time while industrial action takes place."
From Monday, a two-week ban on overtime will begin, during which Mr Claassens said trains will likely be forced to run at a reduced timetable.
On the South Coast line, Wednesday's strike action means trains will run to a weekend timetable and buses will be added to assist trains between Wollongong and Thirroul.
Buses will replace trains between Port Kembla and Wollongong.
Transport for NSW recommended people drive where possible and allow for heavier-than-usual traffic on the road.