Hamilton Conservation Authority has put the city on a flood watch.
""Residents should continue to exercise caution near all water bodies and structures such as bridges, culverts and dams at this time," the statement said. "Elevated water levels, faster moving water, rapidly changing levels and flows, and slippery banks may pose a significant hazard."
City crews are working around the clock to deal with the wet weather, general manager of Hamilton Public Works Dan McKinnon said.
This comes after Environment Canada issued a special weather statement for Hamilton.
Between 40 and 70 mm of rain is expected to fall by tonight, the federal weather agency said.
Dundas, Ont. was particularly hard hit by the weather.
According to Jonathan Bastien, a water resources engineer at Hamilton Conservation Authority, 65 mm of rain fell in the town today.
Dundas, Ont., which is north of Hamilton, is situated in a low-lying area near the Niagara escarpment. These geographic conditions have left storm drains clogged and Spencer Creek swollen.
"It's a ravine," McKinnon added. "It's a bit of a floodplain there so when you get heavy rains like this, it just, wave after wave, you see the accumulation of water there so it's just the natural features there that are contributing to this."
Hamilton police said they started receiving calls about localized flooding before 8 p.m.
One Dundas resident was surprised to find water rushing down her street.
"When I came outside and looked, it wasn't too bad," said Ginette Ipperciel-Berniquer. "About 15 minutes later, I came back outside and it was crazy, you couldn't see the street. We've seen it before, but not this severly."
Police were forced to shut down the main thoroughfare between Dundas, Ont. and Highway 6 due to waterlog on the roadway.
The inclement weather triggered a mudslide, which closed King Street East between East Street North and West Street, said Hamilton Police Staff Sgt. Oliver Mann.
The flooding trapped a driver in his car along Cootes Drive.
Hamilton fire rescued the man from his vehicle around 7:30 p.m., spokesperson Claudio Mostacci told CP24.
According to the conservation authority, flooding is uncommon in Dundas, Ont. at this time of year.
"Dundas usually gets 80 mm of rain in the month of April," said Bastien.
Overflowing water conditions are also reaching the Woodward Wastewater Treatment Plant, the City of Hamilton warned.
Public transit is experiencing delays due to the heavy rain. Several HSR buses have been re-routed due to road closures.
#HSRAlert: We're blocked at Dundas & King. #HSR5 buses to Head St will use West St in both directions. #HamOnt pic.twitter.com/cK6mlH8Fz7At 9 p.m., Alectra Utilities which supplies power across the Hamilton area said more than 400 residents in Ancaster, Ont. are without power.
— HSR (@hsr) April 20, 2017
Halton police are also reporting localized flooding in Burlington.
Crews have closed the North Service Road between Skyview and King Road due to hazardous conditions.
"With the amount of rain coming down, water ways are swollen and moving extremely fast," Halton police said in a Tweet.
The rain is expected to taper off before midnight, Environment Canada says.