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 short time after being stabbed the boy, 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.
A number of teenagers in the area were spoken to by officers but no arrests have been made.
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 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.
A weapon had not been found and investigators were yet to identify a person of interest, he said.
"We're interested in speaking with members of that second group, to assist us with our inquiries.
"Youths walking around the place with knives is not on. We constantly have operations to take knives off the streets," Insp Zdrilic said.
A crime scene has been set up, with police urging anyone with information or footage from the area at the time to get in touch.
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