An Education Review Office report on its visit to the school in March, released in early May, noted the school's roll had dropped dramatically since the office filed its last report in 2015.
Then, the school had 120 pupils compared to the 76 at present.
Of those, close to 50% identified as Tongan.
Mr Roundhill said the drop could be attributed to several factors, including the demographics of Oamaru's population and its central location, which meant it was sandwiched between Fenwick, Pembroke and Ardgowan School zones.
However, its roll had grown over the past several weeks - a trend he expected to continue.
"We would expect to be closer to 90 or 100, somewhere in that range, by the end of the year. I would see the (ethnic) balance staying very similar to what it is at the moment. We're seeing more ethnic groups join our school and we're getting quite a diverse range of students."
The report noted that Pakeha pupils "were not achieving as well as their peers" in writing and maths, and that "school leaders and teachers are yet to effectively address this disparity".
However, the school was praised for its overall achievement levels between 2016 and 2018, when 75% of pupils achieved at "the levels expected by the school" in reading, writing and maths.
Mr Roundhill said the school continued to work with pupils in "target groups", and was otherwise pleased with academic performances, given the school's "large turnover".
"I believe the achievement of our school is very good. Children who start with us or have a longer time with us generally do very well."
He said the school ran accelerated learning programmes for pupils, which the report said was lacking in the past.
"We use a lot of teacher aides to support students in their environment. Teachers are double-teaching ... so the teacher aide is doing the teaching, as is the teacher. A lot of effort is put in to teach each child to make those differences."
In November 2017, Craig Smith was appointed school commissioner to handle staffing decisions after it was placed in statutory management, which was lifted at the end of the 2018 school year.
Mr Roundhill said the school's board had been diligent in its approach to its work.
"We're rebuilding at this stage. We've got a brand new board and they're working very hard to get us in the right directions we would like to be going and changing some of the things that we're doing."
