The new pedestrian underpass that links the underground platforms of the Perth train station is leaking.
Construction company John Holland, which built the underpass as part of the Perth City Link Rail Alliance, has been called back to rectify the problem.
It is believed the leak could be coming from an improperly plugged dewatering hole.
The underpass, a key element of Perth's $744 million City Link project, has been open for less than a year.
It partly runs under heritage-listed buildings on Wellington Street.
Public Transport Authority spokesman David Hynes said testing was done before the underpass opened in December last year.
With no significant rainfall, no major issues were identified.
"As soon as the leak was discovered, we advised John Holland to resolve the problem," he said.