Sydney Trains has blamed significant delays of up to an hour, cancellations and frequent stoppages today on "an IT issue across all transport agencies".

Passengers first began to clog the CBD's train stations last night as urgent overhead wiring repairs at Wynyard Station led to the slowing of train services.

Today city services, including indicator boards, real-time apps and train running information were left unavailable or incorrect as the issues continued.

"This has affected key operating systems that provide visibility of trains and their locations," a Sydney Trains statement said.

"Technicians are working to resolve the IT issue with systems starting to return to operation across the transport network.

"We apologise to customers caught up by this issue and would like to assure them that we are working hard to maintain services.

"Buses are supplementing services at some stations including Campbelltown, Glenfield, Penrith, Olympic Park and Richmond."

Earlier, services on the city's T1 North Shore line were significantly affected by the work with delays of up to an hour.

Some of the services were also been cancelled or terminated at earlier stations, and various trains experienced infrequent stopping between stations as the problem was dealt with.

It is understood the issues will affect passengers travelling to Sydney Olympic Park tonight for the Bledisloe Cup and the Sydney Swans-GWS AFL game.

Despite that, Sydney Trains has assured its customers that special, direct services will run from Central to Olympic Park on top of T7 Olympic Park line trains.

Earlier, Sydney Trains urged commuters to "delay non-essential travel, utilise local buses or make alternate travel arrangements if possible".

Some commuters affected by the travel delays took to social media today to voice their anger at the delays, with some saying they had boarded the wrong trains because of the incorrect information available.

"People are on their phones in the Sydney Trains quiet carriage, cancelling appointments, calling their employer to say they'll be late," one commuter posted.

"Absolutely stuck of being stuck on a train because of delays. Absolute disgrace for a major city," another said.

"Making everyone late and waste their weekends all the time."

NSW Labor has called on Sydney Trains and NSW Transport Minister Andrew Constance to assure commuters that trains will be running to normal timetable by this afternoon with major sporting events taking place.