As the socialising progressed, Pledger-Davies realised his tobacco packet was missing, and accused Wirihana of stealing it. Davies later joined his son, and in the ensuing confrontation took out a folding pocket knife and stabbed Wirihana once, Stuart said.
The blade went 13cm deep and pierced his heart.
Karangaroa was inside with the couple's six children when she heard her partner call out "Babe...babe", then a "thump" as he collapsed into a car in the driveway.
"When I went out he was still breathing a little bit, and then he wasn't. I saw him take his last breath."
She admitted she had earlier stopped at an associate's house and bought methamphetamine to share with some at the party, including Davies.
Another witness and long-time friend of Wirihana, Nigel North, argued with defence counsel Eric Forster over who the aggressor was.
Forster argued Wirihana had swung a punch at Davies just prior to the stabbing.
"Bruce was in no state to swing," North replied.
Forster then accused North of "lying". "What you say can't make sense."
He earlier told the jury his client was concerned for his son's safety and had pulled out the knife only to stop and "scare" Wirihana.
"Even the use of fatal force can still be reasonable self defence," Forster said.
The trial before Justice Christine Grice is set down for one week.
