A top Auckland girls' school is investigating an alleged incident where up to seven underage girls were caught drinking outside the school grounds.

It's understood staff at Epsom Girls' Grammar were called by a parent after a group of prefects, including some who were meant to be at the school hostel, were seen drinking vodka at Mt Eden domain last Friday night.

Principal Madeline Gunn said there had been an incident, which the school was investigating fully.

"We do have a process in place to deal with the whole issue in a restorative way," she said. "I can't say anything more than that as decisions have not been made yet."

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A parent spoken to by the Weekend Herald said she was concerned the girls were not being dealt with, and she did not want her child modelling herself on such behaviour.

"I just think it's a crying shame that a school of that calibre in Auckland city is letting them carry on."