Police officials said the incident happened at the house of one Ranveer Kumar Singh Bibiganj mohalla under Sadar police station of Muzffarpur town.
The labourers have been identified as Sonu Kumar and Manoj Kumar, both residents of Patahi village on the outskirts of the city.
Station House Officer (SHO) of Sadar police station, Anjani Kumar Singh said that the labourers entered the underground septic tank built at the under-construction house to finish some work, but failed to come out.
Prima facie it appears that both either died due to lack of oxygen or some poisonous gas substances that may have accumulated in the pit, Singh said.