More than one million Montreal residents are being told to boil their drinking water after a malfunction at Canada's second-biggest filtration plant.

The malfunction resulted in brownish water gushing from fire hydrants and an unpleasant aroma wafting over parts of the city on Wednesday.

The advisory applied to most of Montreal. City spokeswoman Valerie De Gagne and others describe it as unprecedented.

The advisory applies to 1.3 million people and is expected to remain in place until at least Thursday morning.

Residents are advised to boil their water for at least a minute, or to use bottled water. They also are asked to avoid brushing their teeth with tap water. Water can still be used to wash dishes, clothes or take a shower.