A sharp rise in Sydneysiders travelling to Wollongong on weekdays is turning the tables on the image of the traditional intercity commuter and putting pressure on an already stretched rail line.
Meanwhile, the number of people travelling to Sydney every day for work remained stagnant over the five years to 2006, indicating the Sydney to Illawarra commuter corridor had reached its capacity.
The previously unpublished data comes from the 2006 census, and highlighted concerns about rail and road congestion between the South Coast and Sydney.
The figures show the number of people commuting from Sydney to Wollongong each day jumped 20 per cent over five years to 4300.
While the number of commuters heading north from Wollongong was 19,000 - making it the largest single commuter flow between cities in Australia.
However that number had remained the same since 2001, indicating capacity constraints in the corridor, according to KPMG demographer Bernard Salt.
He had expected to see a 10 to 15 per cent increase in commuters heading to Sydney from Wollongong.
"There should have been another 2000 to 3000 commuters," he said.
This stagnation was most likely due to the commuter corridor being too congested to handle more people travelling daily.
Dr Philip Laird, from the University of Wollongong, confirmed the rail line had reached its limit.
He said the "poor old Illawarra railway line" was on a slow, restricted timetable.
"It's coming under increasing pressure with freight and passengers."
Dr Laird said the line would become more congested once the Sutherland to Cronulla line duplication was completed in the next two years.
"If we don't do anything, the line will continue to get more congested and it will lead to more delays and put pressure on the reliability of operations," he said.
Dr Laird said increased capacity on the railway line could come from the tunnel promised in 1998 from Waterfall to Thirroul and completing the Maldon-Dombarton line for freight trains.
Mr Salt dubbed Sydney commuters to the Illawarra "counter-commuters".
"Perhaps this is showing a maturing of the Sydney relationship with Wollongong and Sydneysiders are embracing work opportunities Wollongong has to offer," Mr Salt said.
"From the days Sydneysiders used to sneer at Wollongong, now they're reassessing ... and it makes sense to commute in the opposite direction, (or) counter-commute."
Mr Salt said the discovery of the increase in counter-commuters was interesting as he had expected that, rather than commuting, people would move to the South Coast for the sea change and find work once they were here.
"It suggests people are reluctant to move down, maybe they've got family commitments in Sydney and they don't want to move," he said.