He said the fire was located at the front building of the wananga.
"It's sort of one big building but there were multiple offices and classrooms and a library on the end of it.
"It was approximately 60m by 20m or 30m. It was a big building with a lot of infrastructure in it."
Mr Rankin said it took about an hour to get under control.
"We also had relief crews until the early hours of the morning to dampen down hot spots."
He said the fire didn't pose a risk to people.
"It was bonus... it was during the day and during a public holiday."
The building destroyed by the fire was under construction, with a conference facility due to open this April, Newstalk ZB reported.
Te Wananga o Raukawa manager Mereana Selby said the building, next to the library, was an old building and it was in the process of being refurbished and extended.
"Luckily for us, there was nothing in this building ... it was completely empty," she told Newstalk ZB.
When she arrived to the scene, Ms Selby said she "got a bit of a shock".
"It was quite a dramatic scene actually."
Although the library had water and smoke damage, the fire was contained before it reached it.
"We are very grateful the fire service was able to restrict the flames from getting in the library," she said.
Several books on shelves were water damaged, however, Ms Selby didn't know the extent of the damage yet.
Ms Selby said she was most grateful there was no one in the buildings yesterday due to Wellington Anniversary day.
She said she felt "hugely disappointed".
"We will go back to the drawing board and see what we can do to make it [construction] happen but it won't be in the time frame that we had hoped."
She said didn't know what caused the fire but she was "trying not to be speculative".
Fire Safety was investigating the cause of the fire.
- Additional reporting Newstalk ZB
