A decision on charges against the boy could be several weeks away, Mr Brown said.
The incident raises the question of "reasonable force" - how far a person can go when they find an intruder on their property - and an expert said a conviction was unlikely if the 15-year-old was defending himself, his father or his home.
Mr Brown said the boy and his father found the intruder in the garage of their home in Alan Rd, Henderson, just after 1am.
"A scuffle ensued, during which the 15-year-old son stabbed the intruder."
The 19-year-old's injuries were serious but not life-threatening. Last night, he was in a stable condition in Auckland City Hospital.
Mr Brown said it was not known where the knife came from, and he was reluctant to speculate on whether the knife was the 19-year-old's or if the 15-year-old or his father had it in their possession.
"It's too early for us to put the complete story together. Until we speak to him [the 19-year-old], we won't have the full picture."
The 15-year-old and his family returned to the house later in the day after police completed a scene examination. He and his father were interviewed by officers.
The parents did not want to comment on the incident because police were investigating.
"He [the 15-year-old] is obviously genuinely upset, and so is his father," Mr Brown said. "They just reacted instantly to a situation. They are as concerned as anyone else."
Mr Brown said it was possible the case would end up in court, with "one or all three" facing charges.
But Queen's Counsel Nigel Hampton said that if it was a straight case of self-defence or of the 15-year-old protecting his home or family, a prosecution was unlikely to succeed.
"You have the right of self-defence - to defend yourself or someone else using such force as is reasonable in the circumstances," he said.
"That depends so much on the individual factual circumstances, though.
"It comes down to who is armed and who isn't armed, the size and physique of the individuals and, in this case, the size and involvement of the 15-year-old's father.
Mr Hampton said it would be difficult for a jury to convict a teenager trying to protect himself, his family or his property using a weapon - as long as the use of that weapon was reasonable.
"If you try to disarm them and they just keep attacking - it would take a harsh-minded jury to convict someone using a knife in that situation."
Sensible Sentencing Trust spokesman Garth McVicar last night applauded the 15-year-old's actions.
"I couldn't think of a more ridiculous situation," he said. "We should be giving the 15-year-old a medal.
"That's the message we need to send to all burglars."
Mr McVicar said that if the boy was charged, he would be at court to support him.
"Every person in this country should have the right to defend their castle, defend their family and defend their loved ones."
REASONABLE FORCE
2002: Northland farmer Paul McIntyre charged with shooting an intruder stealing a quad bike from his Whangae farm.
Result: Acquitted after a trial.
2006: Penrose gun shop worker Greg Carvell charged with unlawful possession of a firearm after shooting a man who was trying to rob his store and was threatening to kill him with a large machete.
Result: JPs sitting in the Auckland District Court dismissed the charge.
2008: Otara liquor store owner Virender Singh charged after he defended himself with a hockey stick against five drunken teenagers who were trying to rob his store.
Result: Charges dropped.
