Waikato District Health Board spokeswoman Mary Anne Gill said the women were last night in stable conditions.
The grandson escaped with only minor injuries but was in shock.
The middle-aged male driver of the silver sedan and the female back seat passenger, understood to have been from Featherston, suffered minor injuries.
Neighbour Guy Tukiri was watching television when he heard a "loud thump" when the cars collided.
At least six residents and passing motorists tried to assist the people injured in the car while waiting for emergency services to arrive.
Two of the rescuers, who wanted to be known only as Ray and Mary, ran to the red car which came to rest outside their house and found the elderly driver covered in shattered glass.
The woman had suffered a blow to the head and told them she had a sore stomach and neck.
"She was covered in glass, her wrist was out like this (twisted outwards), she had lacerations (on her neck and chest), she had hit her head and she was with her grandson coming back from holiday," Mary said.
"All the lady in the red car could say is, 'it just appeared in front of me'."
The couple said the boy, who they believed to be about 15 years old, was being very brave, but said he couldn't remember what happened.
"I said: 'look love you cry you will feel better', and the poor bugger started to cry."
Ray said the woman passenger in the silver car appeared to have been hit hardest.
"The worst one was the woman in the silver car - nobody realised she was in there at first. She was lying down in the passenger seat. Someone must have eventually spotted her as they were all trying to get her out."
The young truck driver parked his truck on the side of the Great South Rd and sat in shock by the accident scene. The serious crash unit is calling for any motorists who witnessed the accident to contact them.
It is the second accident this week in Waikato in which a vehicle has crashed after crossing the centre line.
Thames man Troy Edward McLeod was killed on Monday morning when his car crossed over into the path of a truck on State Highway 2 near Maramarua.