Bain, now aged 36, is on trial a second time on five charges of murdering his parents, Robin and Margaret, two sisters Arawa and Laniet and 14-year-old brother Stephen at 65 Every St, Andersons Bay, Dunedin, on June 20, 1994.
The Crown says Bain, then a 22-year-old student, shot his mother and siblings in their beds and lay in wait for his father before shooting him from a curtained alcove when Robin came into the lounge from the caravan where he was living.
But the defence say Robin killed his wife and three children before shooting himself after leaving a note on the family computer saying: "Sorry, you are the only one who deserved to stay".
And the Crown says that what appeared to be "fitting" by Bain as he sat on the floor of his bedroom while two officers checked the rest of the house for bodies was faked, while the defence says it was a situation where a young man had just heard officers saying they had found the body of the last member of his immediate family.
The trial before Justice Graham Panckhurst and the jury is expected to take 12 weeks.
Bain is represented by Michael Reed QC, Helen Cull QC, Paul Morten and Matthew Karam while Kieran Rafter, Cameron Mander and Robin Bates are prosecuting.
Craig Wombwell, then Dunedin's chief ambulance officer and a qualified nurse since 1976, was the first ambulance officer to go inside the house.
He told the court he was called in because someone was apparently having a fit.
When he went into the left front bedroom, he saw a male (David Bain) in a fetal position on the floor near the base of a bed.
"I expected to find somebody in a fit. What I saw was a male who was shivering," Mr Wombwell said.
Bain's eyes were closed and, when he shone a torch on them, the eyelids were flickering.
His initial assessment was Bain did not have a life-threatening condition needing immediate intervention.
Bain was not displaying any of the classic symptoms of a person who had just had a fit or seizure so he did not look for signs, Mr Wombwell said.
Earlier, Constable Les Andrew said he had been with Bain in the bedroom when he heard the call from officers downstairs that they had found the fifth body.
It was at that point that Bain started shaking before he fell between the end of the bed and the wall.
Const Andrew said he saw Bain's eyes before he fell and they were normal.
"I thought that was a bit strange because if he was having a fit I would think his eyes would react as well," he told the court.
He said he based that on his previous experience of seeing people having fits.
He believed the whites of the eyes became more noticeable and the eyes tended to flicker a lot.
When Bain fell backwards, he did not appear to hit his head before he ended up on his back.
Const Andrew pulled him out from the small space and placed him in the recovery position before checking his pulse and asking for medical assistance.
To Ms Cull QC, the officer agreed that although he had previous experience of seeing people "fitting", he was not medically qualified.
Sergeant Murray Stapp earlier said the way Bain's limbs were shaking appeared to him to be involuntary but he was in no position to say whether or not the accused could control the shaking.
Former Constable Kim Stephenson told the court he had looked into the room across the hallway from David Bain's room and had seen the body of Robin Bain lying on the floor.
What stuck in his mind was seeing a rifle magazine "sitting on its edge" near Mr Bain's body.
He said he thought the magazine's position was "unusual".
There was blood around Robin Bain's head, some blood seepage and a rifle lay on the floor away from the body.
There was a live round on the floor.
When he went into the room to check nobody was behind curtains across an alcove, he had to step over Robin Bain's legs.
He looked behind the curtains and saw a computer with its screen glowing.
He later looked into the doorways of Margaret and Laniet's rooms and saw their bodies lying on their beds, but touched nothing and did not enter the rooms.
Constable Geoff Wyllie said he had heard the sound of tapping on a window when he and the other three officers arrived outside the house.
Initially, they ducked into some bushes and watched the window for a short time.
Then, when they were having trouble getting into the house, he went to the window and asked the person he could see under the window in the room on the left of the door to open the door but the person said he could not.
Const Wyllie said he walked to the other side of the house and looked in the window to see a firearm on the floor and some-one's hand.
Once he and other armed officers were inside, he and Sgt Stapp moved along the hallway, checking the rooms.
As well as the body of Robin Bain in the lounge, they saw the bodies of Margaret and Laniet, in their beds.
Downstairs, he went through the kitchen, along a passageway and reached another bedroom where he found a young woman's body on the floor.
She was on her knees and bent backwards.
He could see her face plainly from the doorway, Const Wyllie said.
He and Sgt Stapp then went back upstairs but they had to check the house again before the fifth body, that of a young boy, was found on the floor of a room off Margaret Bain's bedroom.
It was apparent there had been "quite a struggle in the room".
