The $1.86 billion Forrestfield-Perth Airport rail link will be completed one year later than expected following a major water leak, but the project remains on budget.
The first trains connecting the city's eastern suburbs to the rail network are now expected to run in the second half of 2021, transport minister Rita Saffioti said on Tuesday.
The leak in a tunnel-to-tunnel cross passage on September 22 created a sink hole and damaged a 26-metre section of one tunnel.
Potential causes include construction defects in the grout block, failure of the join between the tunnel lining and grout, tunnelling through the grout block or vibration from excavation of the cross passage.
Ms Saffioti said there would be no cost blowout because the damage would be covered by insurance.
The damaged section of the tunnel has been stabilised with temporary supports.
Permanent repair options include fixing the damage from inside, or rebuilding the section by removing the old rings and casting the new tunnel lining in situ.
Ms Saffioti said the delay was unfortunate but the project was being built to last 120 years.
"I believe the time added to the construction schedule is a small price to pay to ensure we safely deliver this asset," she said.
Opposition transport spokeswoman Liza Harvey said it did not "stack up to commercial reality" that there would be no cost blowout.
"If you've got construction workers working on a project for an additional 12 months, it beggars belief that they'll be able to stay within budget," she told reporters.
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