A structural engineer says Hong Kong's drainage system had not yet been cleared following Super Typhoon Saola last week and this exacerbated flooding on Thursday night and Friday.

Ngai Hok-yan told RTHK that the Drainage Services Department didn't have enough time to clean up damage caused by the typhoon.

"We just had the typhoon and this typhoon was very heavy....fortunately, the rainfall was not so heavy, but the wind had blown down all the trees and other things, so debris may block parts of the drainage system. That is why when the heavy rainfall came [this week], the drainage system could not fully perform," he said.

Ngai said although MTR stations are designed with flood prevention in mind, drainage problems elsewhere meant they couldn't withstand the amount of water brought by the rainstorm.

"If you go the entrance of the MTR, you will find that you need to walk a few steps up and then a few steps down to go into the MTR station. All those steps, in principle, were already designed for possible flooding. Usually even with heavy rainfall, the water should not overflow and then go into the MTR station," he said.

"But if the main drainage system has been blocked by the debris and is unable to discharge, this area will have flooding."

Ngai added that MTR stations are essentially "very deep holes" and pumps would have to be used to get rid of the water in flooded Wong Tai Sin Station.