University council papers from last month showed between January and the end of May it had spent $1.781 million on the Regent Rd project against a budget of $1.2 million.
The total project - which involved adding 20 rooms and upgrading was completed in May - was expected to cost $3.346 million.
The ballooning costs were due to ''an error made on the cost estimates for this project'', the report said.
In response to questions from the Otago Daily Times, property services director Barry MacKay said: ''The project was more complex than anticipated, including the amount of compliance upgrade required so the current facilities could meet new building standards.
''Together, these contributed to cost pressures that were unable to be controlled within early budget estimates.''
The project came as the university and other residential college owners had spent tens of millions of dollars upgrading facilities in recent years.
Director of accommodation services James Lindsay said the university had been carrying out a ''significant renovation and refurbishment programme'' on its residential colleges over the past five years.
''The university understands the value of being a residential university and providing an outstanding campus environment.
''Upgrading the residential colleges is maintaining that outstanding world-class student experience,'' he said.
The upgrade of Toroa included the creation of a large central common space called ''Manawa'' (heart).
''It is the single biggest and best development at the college since it started 18 years ago,'' he said.
''[Manawa] is already a popular place for Toroa residents to study, relax and develop lifelong friendships.''
The additional bedrooms meant it now had 152 bedrooms.
