The heartbeat of a farmer charged with cutting off a teen's finger in a botched home invasion more than doubled as he fought for his life during a violent confrontation with his burglars.

William Bruce Burr, known as Bill, has told a jury how he was woken to a room full of noise after he was knocked on the head with a wine bottle and voices yelling for the keys to his car.

Burr wears a Fitbit to bed each night and in questioning with his counsel Philip Morgan QC said how it shows him waking at 1.45am on October 1, 2020, and his heartbeat skyrocketing to about 111bpm, from around the 50s.

"I was asleep on my left side and next thing I had a bang on my head and I just turned my head around and there was a torch shining in my eyes.

"I thought the room was full of noises, 'where are your f****** keys, we've got a knife, we're going to knife ya, we're going to knife ya', and 'bang' on the head again."

However, it was when the teen boy hit a bottle over his head, with it smashing, for a third time that it really hurt.

"It was one hell of a hit."

After being hit twice by a wine bottle, he said he swung his arms around in the dark and hit a person, before feeling hair and pulling the person - the teen girl - down towards him.

"We were just getting tossed around .. he was pushing ... [we were] tripping over things. It was just a brawl."

All the while, the teen continued to threaten to stab him if he didn't hand over his car keys.

According to the heart rate on his Fitbit, the tussle seemed to last for about eight minutes, when he managed to turn the light on give up his keys.

He said he knew who the burglar was as soon as he heard his voice, and knew the threat to stab him was real.

Asked by Morgan why he kept fighting, he replied "just fighting for my life because I'm not giving up that easily" but ultimately gave up because "I just simply ran out of puff".

Burr said by then the teen "was in a rage", pacing at the end of the bed before throwing a set of house keys he'd found in the kitchen, hard at his head.

He explained how his wife and daughter moved out of their family home after the second burglary, frightened about what happened.

He was then targeted again by the teen on September 23 - when he broke in and found the keys in his jeans pocket - and had his car taken.

The teen had been charged and granted bail to an Auckland property, then driven down with his girlfriend and her cousins to Burr's property during the early hours of October 1.

His evidence continues.