The police serious crash unit is investigating the incident, which occurred outside the Brydone Hotel on Thames St.
Police and emergency services were alerted about 11.25am on Monday.
A 16-year-old high school student was riding the moped, Sergeant Blair Corlet said.
The victim was initially transported to Oamaru Hospital but was later airlifted to Christchurch Hospital in a critical condition.
As of yesterday afternoon the woman remained in a coma.
Sgt Corlet said police inquiries were ongoing.
The Waitaki District Council said no safety concerns had been flagged to the council recently about the crossing.
A media spokesman said until Monday there had not been a recorded crash there in more than a decade.
The police did not divulge which Oamaru secondary school the moped rider was from.
