The two brothers Jahangir, 24, and Izaz, 22, and their father Yusuf, 50, who have been identified only by their first names, had gone inside the sewer and spent some hours cleaning it. When they did not come out, people called the police and the fire department.
A fireman identified as Mahipal, who was the first to enter the sewer to look for the three people, also inhaled the toxic fumes and collapsed, the police said. All the four were taken to Hedgewar Hospital.
The police said Jahangir died on the way while Izaz died after some time during treatment, Deputy Commissioner of Police (Shahdara) Nupur Prasad said, adding the father and the firemen were undergoing treatment.
Delhi Water Minister Rajendra Gautam on Saturday formed a panel to look into alleged negligence by private sewer-cleaning contractors. "I am making a vigilance committee to monitor such illegal activities. I am shocked to see the courage of these contractors. They do not fear the law," Mr Gautam said.
"Though we have no control on private buildings, we will take stern action against violators," he added.
Last week, three men who were cleaning a sewer at south Delhi's Lajpat Nagar died after inhaling toxic fumes. Four weeks ago, another four men died while cleaning a septic tank in south Delhi's Ghitorni.