Police said the girl was found unresponsive on an Erris St footpath about 10pm last Wednesday by a man taking his dog outside.
He had heard ''shallow breathing'' and decided to investigate.
When ambulance staff arrived, they found the girl ''between status one and status two'', Sergeant Simon Paget said.
''It basically means her condition was life-threatening. She had hypothermia.
''Had she been out there another hour she could have died ... It took hospital staff quite some time to get her core temperature back to where it should be.''
''Had he [the man who found her] not had the presence of mind to check out the noise he heard, there's no doubt that we would have been dealing with a tragedy.''
An 18-year-old female would be charged with supplying alcohol to a minor, he said.
The girl had been a part of a group of young people drinking on the Cromwell golf course, after the 18-year-old bought and supplied them with a bottle of vodka.
After becoming ''extremely'' intoxicated, the young people became separated, and the 16-year-old collapsed.
Police had spoken to members of the group and investigated the source of the alcohol.
It was not clear how the group had become separated, and their recollection of events was ''not that flash''.
''This is a clear illustration of the dangers of people supplying alcohol to younger people ... they need to realise there are very real consequences.''
It was not clear if the girl's parents knew where she was during the evening, Sgt Paget said.
The incident had been ''pretty traumatic'' for the girl and her parents, who had approached the man who found her to thank him.
Cromwell College principal Mason Stretch said the school was relieved the girl, a pupil at the college, was well and had returned to school.
The incident was not related to the school, and was in the hands of police, so he could not comment on it further, he said.
However, the college provided ''plenty of information'', beginning in year 7, about making positive decisions regarding alcohol.
''Students are well informed on the negative consequences if they make a mistake in that area,'' Mr Stretch said.
Cromwell Youth Worker Trust member Marie Roxburgh said she did not think Cromwell had a bigger youth alcohol problem than anywhere else.
The trust did ''a lot'' of work relating to the issue, and had strategies in place to deal with issues that came up, she said.
Sgt Paget said such serious youth-alcohol incidents in Cromwell were ''pretty rare''.
Police in the area had noticed a ''huge'' improvement in youth-alcohol issues since the Sale and Supply of Alcohol Act came into effect, he said.
Although the culture of adults supplying young people with alcohol had changed, the 18-20 age range was still a problem.
''We still find ourselves banging on the same message, and people need to switch on, and it's frustrating that we put a message out there and some people choose to ignore it.''
Four people in the 18-20 age range had been in court in the area recently for supplying youths with alcohol, he said.
Other youth and alcohol-related agencies contacted did not comment.
