The teen is fighting for life in hospital and has undergone surgery after the stabbing in the NSW Central Coast town of Terrigal shortly before 9.45pm on Friday.
A 15-year-old boy has been arrested and charged over the stabbing.
A short time after being stabbed the victim, who was out with friends, alerted a random homeowner on Painters Lane for help, who rang triple zero, police said.
Paramedics treated the boy at the home before he was rushed to Gosford Hospital, where he remains in a critical condition.
The victim was in a group with several other teenagers at the time of the stabbing, police were told.
Painters Lane is about 350 metres from Terrigal Beach, the town's centre and main esplanade.
Detective Chief Inspector John Zdrilic told reporters on Saturday that investigations were focusing on two groups of teenagers reportedly in the area at the time.
"Those two groups met up in the Terrigal area, they appear to have gone to Painters Lane," Detective Chief Inspector John Zdrilic said.
"The 15-year-old who was stabbed was there with two mates, and they met up with another group that could have numbered up to five."
The boy suffered a single stab wound to his left side, causing "heavy bleeding", Det Insp Zdrilic said.
"Youths walking around the place with knives is not on. We constantly have operations to take knives off the streets," Insp Zdrilic said.
About 3pm on Saturday a 15-year-old boy was arrested at Gosford Police Station and charged with wounding a person with intent to cause grievous bodily harm, police said.
He was refused bail to appear before a children's court tomorrow on Sunday.
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