The Barnett Government has used inflated patronage projections for its airport-Forrestfield rail link relative to published projections for other rail proposals.
Transport Minister Dean Nalder last week released a summary of the airport rail link with projections that its three stations would have 20,000 "daily trips generated" when it opened in 2020 and 29,000 by 2031.
But previous projections for Perth public transport projects, including the MAX light rail, were based on daily boardings.
The Opposition argues the "daily trips generated" is effectively a double count of daily boardings - an argument supported by fresh data on the airport-Forrestfield line tabled in State Parliament this week.
Last week, Colin Barnett argued for the airport link when he compared patronage figures with an Ellenbrook line, but used differing measures. "There will be 20,000 patrons using the (airport) rail line every day as of 2020 and 30,000 patrons using it by 2030," the Premier said. "That is compared with the much-debated Ellenbrook rail line, which would have 10,000 patrons."
Yesterday, Mr Nalder said the 10,000 figure for Ellenbrook was in fact "passenger boardings".
Opposition Leader Mark McGowan said the Government misled the public in an attempt to justify its infrastructure choices.
Mr Barnett told Parliament the distinction between the measures was "a trite point".
The Opposition also highlighted catchment assumptions for the airport line that include residential areas within walking distance of Midland line stations, saying it was ridiculous to assume these people would drive 5km to an airport line station.