Three people who fell into a campground's septic tank have died and a fourth man is in critical condition in hospital Saturday.

The incident occurred late Friday evening when the owner of the campground, his son-in-law and another camper went down a four-metre hole near the tank to clear a blocked drain, reported Quebec provincial police.

The three men were overcome by fumes and fell unconscious. A fourth man who tried to save the trio was also overcome by fumes and sent to hospital.

Eight other people who tried to help and two firemen were treated for exposure to the toxic fumes.

The victims' names were not released.

An autopsy is planned this week to investigate whether the deaths were linked to the fumes or if the three men drowned in human waste, said police spokesman Const. Ronald McInnis, who warned that only professionals should fix septic tanks.

St-Jean-Baptiste-de-Rouville is about 30 kilometres east of Montreal.