There cannabis branches lay drying over ordinary household items, and there was also a "considerable" number of firearms including a loaded shotgun at the foot of the bed.
According to the defendant, the pair had recently learned they were to be the likely target of a "stand-over", the prosecutor said.
Johnstone said methamphetamine also seemed likely to have affected Smith's thinking that night.
She was perhaps more "sleepless and alert" than most people to unwanted visitors and was "more ready" to take action against them.
Sometime after 1am, she sensed an intruder on the property, he said.
She armed herself with a rifle, the court heard.
"She knew it was loaded. From her perspective every gun in the house was loaded and ready to go."
Playing on her mind - and the prosecutor quoted - was "the length mongrels will go to get what is not theirs to take".
She saw a person outside, he said, but was unable to see who or what they were doing.
"But even so, she decided to shoot the person," Johnstone said.
"Tragically, hers was what might in other circumstances be described as an excellent shot.
"The bullet went through the man's left arm and carried on through the side of his chest through his left lung lodging in his heart."
But there was no intruder that night, the prosecutor said. She had shot Taylor on his own property.
Police launched a homicide investigation after they were first called to the rural address in South Head about 2am. Photo / Dean Purcell He yelled, "f**k you shot me", Johnstone said.
Smith called 111 for an ambulance but he died before it arrived.
"Ms Smith clearly used unreasonable force," Johnstone said.
Her actions were not to defend herself but to stop someone coming in to steal drugs, he said.
But the "only mistake" was that it was Taylor and not some other person who would suffer the consequences, he said.
The Crown is expected to call just under 20 witnesses during the trial. Evidence started today with Smith's 111 call.
"Naturally you will hear some of the anguish in her voice," Johnstone said.
Defence lawyer Peter Kaye said it was accepted that his client fatally shot Taylor by mistake but there were issues.
"What happened in those crucial moments when Ms Smith had that gun?
"Was she acting to defend herself or another, namely Mr Taylor - the man she loves?"
How did Smith view the circumstances she was in at the time, he said.
"You are looking at it through her eyes."
The 111 calls: 'Amy, what have you shot Danny with?' Smith was excused from listening to six 111 calls that were played to the jury this afternoon.
First she repeated their address for the call taker. Then she said she had shot her partner in the arm.
Smith described Taylor as pale and tried to stem the bleeding as the call taker repeatedly told her to apply pressure on the wound.
"Now, Amy, what have you shot Danny with?"
"I shot him with a .22," she said.
"I thought he was an intruder.
"I didn't realise he had gone downstairs."
Throughout the call a distressed Smith can be heard yelling indecipherably.
At one point she says: "I don't know how to control the bleeding."
"Amy, calm down, you need to keep firm, steady pressure on the wound," the call taker replied.
Smith tried pleading with the dying 52-year-old for help, who can be heard groaning.
"Baby what do I do? Tell me what to do. You tell me. You know, you know what to do.
"Just tell me."
She also tries comforting Taylor.
"I know it's sore. I know it's sore. You gotta bear with me," Smith can be heard saying.
"Stay still please."
There is more yelling on the line before she hungs up and the call is reconnected several times.
The trial, which started today before Justice Mary Peters and jury, continues in the High Court at Auckland.
