Queensland's Liberal National Party has accused the Palaszczuk government of over-estimating the need for its flagship Cross River Rail project.
Deputy Premier Jackie Trad says the $5.4 billion project is a "no brainer" to reduce congestion at the Merivale Bridge, which is currently Brisbane's only river rail crossing.
The government on Tuesday announced it would go ahead and fund the project on its own, rather than wait a couple of years to seek money from the Turnbull government's $10 billion National Rail Program.
But Deputy Opposition Leader Deb Frecklington said the government own rail figures show there is no need to rush the development because passenger numbers are dropping.
Since 2010/11 rail patronage on the southeast Queensland network has dropped from 55 million people to 51.1 million people, according to the Department of Transport and Main Roads.
That's despite Labor's as yet unreleased business case for the project reportedly showing six per cent annual growth in passenger numbers, Ms Frecklington said.
"Labor's claimed six per cent year-on-year growth can't be believed," she said.
"Labor knows its numbers are dodgy. The capacity constraint on the Merivale Bridge won't occur until 2026 at the earliest, so we have the time to get this right."
Ms Trad's office has been approached for comment.