Public and private schools, particularly in the northern suburbs, are stretched to capacity from increasing student numbers with 104 demountables spread across 94 schools.

Enrolments at Coledale, Austinmer and Thirroul public schools have collectively jumped by 172 students in just four years.

The latest Department of Education average class size for mid-sized primary schools is 24, meaning these schools have needed to accommodate more than seven extra classes of students among them.

The vice-president of Austinmer Public's P&C association, Anna Morris, is concerned the school's nine demountable classrooms sacrifice play areas.

"They just cut out playground space, since I've been here they've increased from seven demountables to nine because there's just more kids and they just don't have the facilities," she said.

"We have major capital works issues, we only have one airconditioned class in the whole school, not enough urinals for students and one toilet between 38 staff."

A spokesperson for Wollongong's Catholic Education Office (CEO) attributed the baby bonus, along with recent housing developments in the Illawarra's north, for pushing up enrolments.

"Enrolments peak and trough but this is an ongoing spurt from the baby bonus boom reaching school age," he said. "We don't like turning anyone away but we've had a situation where demand outweighs places."

To cope with demand, the CEO works with parents to find places in other Catholic schools in addition to asking parents to defer their child's first year of school where possible.

The CEO expects between five and 20 new Catholic schools to be built in the Wollongong diocese in the next 20 years based on demographic projections.

The enrolment influx seems to be isolated to northern and central Wollongong with Department of Education figures showing enrolments at Albion Park, Oak Flats and Dapto public primary schools dropping by 241 students between 2011 and 2014.

Demountable classrooms have become an election issue with NSW Labor announcing a $100 million demountable replacement scheme as an education election promise.

Kiama Labor candidate Glenn Kolomeitz claimed Liberal government funding cuts had stunted previous attempts at replacement schemes.

"The Liberals bundled Labor's demountable replacement program into the wider school capital works budget - then tore away $280 million.

"Demountables were always meant to be a temporary fix ... students at Albion Park High, Berry Primary School and Gerringong Primary School are studying in demountables which are 19 years old."

brianna.parkins@fairfaxmedia.com.au