"Secondly, show producers have prescribed guidelines surrounding acoustics and light intrusion which meant the doors could not remain open throughout the performance."
Mr Fell said more than $1 million had been spent on the Opera House to upgrade its seismic strengthening, fire and access compliance, refurbishment of the seats and a new roof.
"While council had considered installing an air-conditioning system, the capital costs could not be justified for several evenings a year throughout the summer months.
"In the longer-term the council will consider a system for chilling incoming air, but in the meantime work will begin on options for more efficient ventilation of external air while keeping light, sound and fan noise to acceptable levels. "This should be resolved within four months.
"I am sure it can't come soon enough for many present last Saturday evening," he said.
Mr Fell said apart from the temperature, the audience had been "captivated" by performances from students who attended the New Zealand Opera School.
"It was an incredible opportunity for Whanganui people, and visitors to our district, to experience opera in a purpose-built facility and hear the stars of the future."