Five other labourers also fell unconscious in the incident that was reported from the plant in Kondli at around 11 am.
The deceased have been identified as Moolchand (45) and Dileep (42).
Moolchand had entered the sewage plant with Dileep but they soon fell unconscious after inhaling poisonous gases from the tank. Five other labourers entered the sewage line but they too fell unconscious.
Other workers managed to pull all the seven out of the sewage line and rushed to Lal Bahadur Shastri Hospital where Dileep and Moolchand died.
Police said the condition of the other five is stable.