Eyewitnesses claimed the bus was travelling at high speed towards Nadzab when it came upon another vehicle in front. The driver tried to veer away from it when he lost control.
The 25-seater PMV bus, which operates between Lae and Nadzab in Morobe Province, ran off the road near the Flour Mill at Six-Mile and nosedived into the nearby drain.
The driver and five others died instantly from the impact while other passengers were rushed to the Angau Memorial General Hospital.
Police were at the scene to assist the casualties.