"We just thought he had hurt himself, but it was much worse. She'd already passed away by the time we got there but I was told to keep doing CPR," said Stacey Ball, 20.
After "it all sank in" that she had died, her partner looked as though he was about to faint.
Deanne Ball said the boy always came across as a polite young man.
"They come over here to have coffees and he's always been well mannered. It's just one of those things that you read in a book - it was completely out of the blue.
"I never saw it coming. He's always polite and out helping... with the firewood and out on the farm."
Family members were last night supporting the victim's partner, she said.
The victim worked as a caregiver, a job which took her around the country. She had recently been to Nelson to look after an ill person.
Deanne Ball said she was a very bubbly, kind and caring person. "But if she had something to say, she'd say it."
Police said a firearm had been recovered from the scene and family members were being interviewed.
Last night, the house was under police guard and the vehicle the boy was in had been taken for examination.
The woman's body was to remain in the house overnight.
Deanne Ball said she wasn't coping with what she and her daughter saw and she was still in shock hours later.
"I haven't seen anything like that before in my life. It was horrible.
"The ambulance people kept telling her [Stacey] to keep going [with CPR]," Deanne Ball said.
"I was worried about Stacey, to be honest, but she handled it better than me. I was much more horrified than her."
Stacey Ball worked on her neighbour or five to 10 minutes until a rescue helicopter was able to reach them and paramedics took over.
She said she knew what to do because she had taken a first-aid course for her diploma in professional cookery at Western Institute of Technology.
"It certainly changed your Sunday afternoon - we were just doing the gardening and this happened ... I'm still in a state of shock."
Stacey Ball said she was "doing okay. There's not much I could have done about it. I just did what I've learnt and what the ambulance people were telling us."
Detective Senior Sergeant Grant Coward, officer in charge of New Plymouth CIB said: "It is premature for people to speculate on what happened and who might be involved. We have been talking with people who were at the house to find out more.
"This is a stressful and emotional time for people who were present and we are mindful of this."