The Essex Rocks seem to have gone to the beach early and this was a fifth successive defeat with 17 goals conceded.
Rory Williams gave the Hawks a 16th minute lead with a fine strike from the edge of the area.
But East Thurrock equalised after 33 minutes when an effort from Reece Harris was deflected past home keeper Ryan Young.
So, 1-1 at half-time and it took the Hawks until the 61st minute to re-take the lead when a free-kick was headed in, although there was some confusion as to whether it was an own goal or if Matty Tubbs got the final touch.
There was no doubting that Havant`s third goal 15 minutes from time was definitely a Ryan Scott own goal, and 2 minutes later, top scorer Jason Prior (pictured) made it 4-1 from close range.
Six minutes from time Theo Lewis headed in the Hawks fifth and Prior grabbed his second and the Hawks` sixth a minute from time.
The final day scenario is now simple – Dartford have to better Havant`s result at home to Concord Rangers at already-relegated Bognor Regis Town as the Hawks now have a three-goal advantage.