Mr Bullen said hundreds of thousands of dollars would need to be spent on fences, toilets and at least two relocatable classrooms at the Tenby St school site this year to cope with growth.
But it was just "money down the drain" because a new $7 million school was planned at Scurr Heights.
Difficulty getting information from the ministry about construction timelines for the new school made him feel nervous the ministry might not be committed to it.
A new primary school in Frankton seemed to be progressing much faster, he said.
"The school will spend $80,000 in the next couple of months for fencing and upgrades to toilets to take care of high dependency kids..."
"The ministry will be paying for that but it is money poured down the drain because we are not in the new school, which will be built to facilitate that [special needs facilities]," Mr Bullen said.
"...We need confirmation our new school is going to proceed and we need that as soon as possible. Without that, we cannot meet the construction deadline..."
"We are physically going to start running out of room because the sports fields are going to have to take these classrooms," Mr Bullen said.
Two or three relocatable classrooms were needed before the end of this year to cope with roll growth and to provide an 18:1 teacher ratio for new entrants.
The classrooms would be trucked from Dunedin, where rolls were dropping, and could cost an estimated $350,000 or more, Mr Bullen said.
The Wanaka school roll was 378 at the end of term one and increased by 27 on Monday, when the second term started.
The board needed progress on paperwork submitted to the ministry in March so it could start earthworks to meet the term four deadline to begin using the classrooms.
The earthworks should take place in the winter to avoid spring dust-storms and longer delays, Mr Bullen said.
Obtaining infrastructure, especially an adequate power supply, would also pose problems, he said.
The school's administration and staff areas were also under pressure and the urgency was being compounded by the imminent arrival of hundreds more babies in the Upper Clutha region, Mr Bullen said.