Perth commuters are jumping off trains in record numbers, with more than 1.26 million fewer passenger journeys in the second half of last year compared with 2012.
The Opposition has blamed the dramatic fall on carriage overcrowding.
"This is what happens when you don't order enough carriages soon enough," shadow transport minister Ken Travers said.
The new figures from the Public Transport Authority show the six-month patronage dropped from 32.6 million to 31.34 million. Numbers dropped on the Armadale and Fremantle lines in each month.
The slump comes after several years of remarkable growth for public transport in Perth. In just six years, annual patronage jumped from 100 million to nearly 150 million people in 2012-13.
At the time, Curtin University sustainability expert Peter Newman said the growth was "remarkable, higher than predicted" and "difficult to see how it can be sustained".
But Mr Travers said overcrowding was discouraging people from using trains and the lack of planning meant extra carriages were not ordered soon enough.
"Even when all the new carriages do arrive, it still won't be enough," he said. "And if commuters are choosing to use their cars, it's adding to congestion."
The Australasian Railway Association has reported that an average train carriage of passengers takes 525 cars off the road.
A transport authority spokeswoman said there were several factors that could have contributed to the fall in patronage, including the shutdowns in July and September as part of work on the Perth City Link project.
"There was very bad weather between July and September - a factor we believe impacts passenger decisions about whether to take public transport," she said.
"Our figures show there has been less discretionary travel on our system, as the number of passengers using FamilyRider tickets and DayRider tickets is down.
"Further, unemployment and office vacancy rates are up, so we believe less people are travelling into the city on a typical work day."
The figures also show an unexpected rise in ferry patronage in December to 47,913 passenger journeys - the biggest monthly figure since October 2011.