The Home Affairs Department on Tuesday called on the public not to get close to either side of the Shing Mun River in Sha Tin after the water level at the Lower Shing Mun Reservoir reached the warning threshold.

The department said it had notified residents in housing estates near the river, such as Mei Lam Estate, May Shing Court, Mei Chung Court and Granville Garden.

It said it had also alerted the Sha Tin District office, Drainage Services Department and the police based on an emergency mechanism.

The department added that police would cordon off each side of the river.

Separately, some people headed to work as usual despite a black rainstorm warning being in place.

A caregiver surnamed Chan said she was not supposed to go to her work in North Point before the signal was lowered, but left her home in Tseung Kwan O at around 9am to avoid any crowds.

The MTR closed several exits of stations due to flooding.

A woman surnamed Mak said she had to make a detour to get to her office.

"I cannot go directly to my office. I need to take a longer route," she said.

"The MTR has made some announcements, but I wish they would also announce this inside the train compartments. I only found out about the closure when I reached the exit."

Teacher Leslie Richardson said she enjoyed going out in the storm because it was helping to cool down the city.

"I stayed up late last night listening to the storm, but enjoying it. I like this. I'm from Los Angeles. We don't get storms like this often," she said.

"The other thing that's very nice about Hong Kong is that people's residences are built right on top of the MTR. So despite whatever the weather condition is, with the MTR system and the infrastructure, it's very easy to continue to get around Hong Kong."

A couple from California, however, were not so impressed with the storm which came on the last day of their visit to Hong Kong.

"We've never really seen this much lightning before, ever. California's really dry and this is a big shock for us, how much it's raining here and how much lightning there is," said Agustin Valadez.

"It's a little scary because it just came out of nowhere and it was very intense when we woke up today and it was still lightning and thunder, so it was a little scary for us."

The observatory said it recorded more than 11,500 strokes of lightning between midnight and 11:59am.