The school has received approval and funding from the Ministry of Education to refurbish the building.
"You didn't want to have a shower in here because the water pressure was terrible," principal Dan Reddiex said.
The gym, with its draughty changing rooms, was built in the 1970s and had remained relatively untouched since then.
However, a $250,000 capital expenditure grant from the ministry would allow the school to refurbish the changing rooms and office area, and repolish the main gymnasium floor.
The funding would also be used to renovate the foyer, making it a place to display trophies and photos of past and present pupils who have gained national recognition.
"It's going to be an area of celebration. This year, we've had a number of New Zealand representatives we want recognised in the foyer."
Although it was hoped the gym refurbishment would help boost sporting success among pupils at the school, Mr Reddiex said it would also boost academic achievement.
"Research has showed an interesting link - that boys who engage in extracurricular activity, such as sport, do better in NCEA."
Construction had started, and it was hoped the refurbishment would be completed by Christmas.
The school was also working through a proposal to remodel the music department, he said.
