West Australians are still no closer to finding out when they'll be able to catch a train to Perth Airport, more than two years after the long-awaited rail line was due to open.

The Forrestfield-Airport Link has been more than a decade in the making and is now nearing completion.

But exactly how long the final stretch will last remains unclear, with the government refusing to give any certainty beyond an opening some time this year.

The opposition said that was because the government had something to hide and called on Transport Minister Rita Saffioti to provide a date soon.

Uncertainty around the line's opening comes amid chaos at the airport, with flight cancellations and car parks filled to the brim.

After issuing an initial warning that travellers would not be able to book bays between Tuesday and Thursday, Perth Airport said more bookings were now becoming available.

"Due to the trend in strong, FIFO-related bookings, Perth Airport expects the demand for car parking to continue," a spokesperson said.

"We recommend booking at least a week in advance to secure a parking bay and access the best available rate."

Timelines as soft as tunnel sand

Had the rail line been opened on schedule, it might have helped relieve some of the current pressures.

But that date has gradually drifted, from 2020, eventually landing around the middle of this year.

In early May the project's website told visitors: "First trains will operate on the new Airport Line in the first half of 2022."

Replies to comments on the project's Facebook page shared a similar message, up until their tone changed on June 8.

"First trains will be operating on the new Airport Line in 2022," the new reply read.

"The opening date will be provided once the testing and commissioning phase is nearing its end."

That phase looks set to continue until at least the end of this month, with three "emergency readiness" exercises being planned for 17, 20 and 30 July.

Once the commissioning phase is complete, driver training will need to follow.

When the Mandurah line was being opened in 2007, it took just over a month from driver training starting for the line to open.

If those emergency exercises came towards the end of the commissioning phase, that could make an opening before September unlikely.

Soil, safety and COVID added to delays, Minister says

Transport Minister Rita Saffioti was questioned about the project's timeline during budget estimates in late May, and told Parliament there had been several issues throughout the line's lifetime.

"First of all, tunnelling through soft sand, water and gravel on clay was pretty difficult," she said.

"In September 2018, there was a significant disturbance to the tunnelling, which had an impact on the timeframe."

Delays were also attributed to safety concerns raised when a worker was left with a traumatic brain injury after an incident on-site that led to a $200,000 fine for the construction company involved. 

"The government made a clear decision that safety was the number one priority and, as a result, there were extensions to timeframes early in 2018, 2019," Ms Saffioti said.

"Subsequent to that, the project was hit with many of the challenges faced when building a house in the suburbs, such as impacts on the supply of vital equipment.

"The project has been impacted by COVID, labour shortages and general supply chain disruptions."

Ms Saffioti also said the trains would cater for between 80 and 85 per cent of all flights leaving Perth Airport, with passengers charged a "very affordable" fare capped at $5.

Parliament was also told that patronage across Transperth's network remained below pre-COVID levels, and was not expected to recover until the 2023-24 financial year.

'Poor management', opposition says

The opposition's Metronet spokesman, Tjorn Sibma, said the lack of a firm timeline could mean the government had encountered undisclosed issues during the testing phase.

"It's not hard to work out a commencement date. What tasks are left to do, and how long will these tasks take?" he said.

"Given the nearly $2 billion price tag of the Airport Line, West Australians deserve to know the answer to these simple questions.

"If the government won't answer, they clearly have something to hide."

In response, a state government spokesperson said tunnels and stations "include numerous safety features that undergo rigorous testing prior to opening — this work is ongoing".

"Confirming an opening date is subject to the successful completion of these activities. We expect to be able to provide an update in coming weeks," the spokesperson said. 

Mr Sibma also rejected the government's claims that COVID-19 could be blamed for the delayed opening.

"While COVID has been a complicating factor, it is a lazy, get-out excuse the McGowan government relies upon whenever it wants to shirk responsibility for its continual underperformance," he said.

"It is just poor project management at the end of the day."

A sinkhole, sand and other snags

Planning for the Forrestfield-Airport Link first began in 2008.

A key promise in the lead-up to the 2013 election, it was initially meant to be finished by 2018, although that was later pushed back by another two years. 

Before construction had even begun, outspoken Liberal MP Rob Johnson suggested in 2015 the project should be scrapped altogether, as the government struggled to secure a GST deal to sure up its finances. 

Work on the ground officially began in November 2016, but the project hit a major snag in 2018, which started when "ground disturbances" halted one of the tunnel-boring machines being used on site. 

It was two months before tunnelling work could resume, but Transport Minister Rita Saffioti insisted the delay would not delay the project beyond its scheduled 2020 opening. 

Later that year, concerns were raised that the project could be affected by a national shortage of skilled rail workers. 

Around the same time, workplace safety concerns were raised when a worker was knocked unconscious by a hose pipe and left with a traumatic brain injury.

That led to the company involved being fined $200,000. 

A water leak, which led to a sinkhole, also contributed to delays. 

When the line is completed, 20,000 passenger trips are expected to be made each day.

Services between High Wycombe and Perth will operate every 12 minutes during peak, and every 15 minutes off-peak.

The line's opening will also affect services on the Midland and Fremantle lines.

Trains are planned to run every six minutes from Bayswater to Perth and from Claremont to Perth during peak hours.

Services from Fremantle to Swanbourne and from Midland to Ashfield will run every 12 minutes during the peak.