The victim, 22, who had been in the flat just a few hours, was rushed to hospital and flown to Wellington for emergency surgery after the axe was thrust into his sternum.
He received a cut to his heart, a punctured lung and fractures to two ribs.
At his trial in Napier District Court this week, Gillies denied charges of aggravated burglary, wounding with intent to cause grievous bodily harm, assault with a weapon and common assault. He was found guilty on Tuesday.
Matenga, of Flaxmere, earlier pleaded guilty to six charges, including one of wounding with intent to cause grievous bodily harm and was sentenced to five years and 10 months in jail.
Following Gillies' guilty verdict, Detective Sergeant Darren Pritchard described the attack as a "brutal incident that deeply affected the victims both physically and emotionally".
At Matenga's sentencing, Crown prosecutor Steve Manning said victim impact statements reflected the "lawless nature" of an intrusion which had serious and long-term effects on the victims.
Judge Rea said Matenga's degree of violence, despite occurring while on prison release conditions, was surprising given he had only one prior conviction for violence.
Gillies will be sentenced in September.