Classrooms in Australia’s primary schools are becoming increasingly crowded and teachers have warned that learning will suffer unless pupil numbers are reduced.

New South Wales and Western Australia have the nation’s biggest primary classes. Primary schools in WA have an average of 27.3 children in each class while in New South Wales the figure is 26.9.

The Australian Education Union says young children’s learning in particular is being compromised by the increase in class size.

Class sizes in Victoria and Queensland are also rising. Over the past five years, the average number of pupils per class in Victoria has jumped from 23.4 to 26 and in Queensland from 23.8 to 25.4.

Conservative governments in all but New South Wales have cut spending on schools while in Victoria the government slashed teacher numbers by 8,000 - more than a fifth of the workforce - after taking office late in 1992.

For the first time last month, the Victorian government released figures showing average and maximum class sizes for all state primary schools.

The data released gave a snapshot not only of pupil numbers but also of each school’s share of the government’s $1.15 billion (pound;450 million) spending on primary education.

The figures showed that some schools have classes of up to 38 pupils, with more than a third having at least 30.

Although school funding is based on a formula of one teacher for 21 pupils, principals decide how the money is to be split between classroom teaching, specialist provision such as art, music and physical education, and administration.

Phil Gude, the state education minister, said the issue of class sizes was a matter of great concern to parents and the subject of heated and emotive debate.

“The government is happy to participate in that debate but insists that it be from an informed and factual basis,” Mr Gude said.

But he came under attack from critics in the Opposition Labor party and from school principals. They said he was trying to shift the blame for increasing class sizes from the government to individual schools.

Bruce Mildenhall, Labor’s education spokesman, said the figures had put principals “in the hot seat”. They faced a dilemma where they had to choose between offering specialist courses or having reasonable class sizes.

“The principals are the ones who will have to face the parents, not Mr Gude. Parents will be very concerned about what these figures show and will be demanding answers,” Mr Mildenhall said.

A spokeswoman for the AEU condemned the Victorian government for allowing class sizes to increase. She said learning results increased significantly once the number of pupils was 25 or lower. The optimum level was 20 in a class, she said.