Thousands of tons of water are being shipped in by road
"Benzene levels were 108 times above national safety levels," said China's Environment Protection Administration.
The contamination after an accident at a chemical plant is expected to pass through Harbin on the Songhua river for the next two days, officials said.
Some schools and businesses have closed and flights out of Harbin are sold out.
"Everyone wants to leave Harbin and it is very difficult to buy tickets," a factory manager told Reuters.
Benzene is a highly poisonous toxin that is also carcinogenic.
Fifteen hospitals have been placed on stand-by to cope with possible poisoning victims.
Officials are also on alert in Russian towns further down the river.
More than 16,000 tons of drinking water is being brought into Harbin by road, the Chinese state news agency Xinhua said - though this is less than Harbin's residents normally use in a day.
The government initially said the stoppage would last four days, but a water company official has told the BBC there is no set timetable for the resumption of supplies.
BBC Beijing correspondent Louisa Lim says residents of Harbin distrust government statements, having originally been told the stoppage was for routine maintenance.
The initial announcement of water stoppages led to panic buying of water and food, exhausting supermarket supplies and sending prices soaring.
"The city was full of ridiculously large queues. People were buying water in massive quantities," English teacher Craig Hutchinson told the BBC News website.
Other residents told the BBC they felt more inconvenienced than worried.