A farmer has described how and why a teen burglar had part of his finger chopped off during the burglary of his home.

"I said to Shaun, 'chop it off'. Shaun got it and chopped it off," William Burr told a jury in the High Court at Hamilton today.

In taking the stand this afternoon, Burr snr said he knew the teen was hiding a knife under his body as he lay face down on the ground.

Despite repeated requests for the teen to put both his hands out in front of him, he refused.

His son then arrived and they noticed the boy trying to stand up so he instructed Burr jnr to "smack him down".

"The bastard had bloody got up. I said 'smack him down, Shaun'. He smacked him, beautiful, right in the face.

"He dropped to the ground and Shaun kept on punching him."

However, he became nervous about where the knife was and repeatedly asked him to put his hand out.

"I said 'we'll cut your hand off'. He kept his right hand out but he still wouldn't comply so, 'get your hand out'.

"I gave Shaun a knife and I say to Shaun 'cut his bloody finger off' unless he gets his hand out.

"Shaun got the knife and just gave it a little cut at it. [Teen] come on [teen], stick your hand out, get that left hand out' and he wouldn't.

"So I picked up a bit of wood and I say to Shaun 'chop it off'. Shaun got it and chopped it off."

Asked what the boy did in response, he said "he just lay there".

He described the teen as "fearless".

"He told me he didn't mind being shot."

Earlier, the court heard how Burr snr's heartbeat more than doubled during a violent scuffle when woken up by the pair.

He said 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 for a third time - the bottle smashing - 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 said he'd 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 he 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 had 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 Burr's jeans pocket - and his was 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.