According to data obtained by Mr Park through freedom of information, 1263 demountables across the state have been at their current sites for more than 10 years, including 806 that have been there between 15 and 20 years.

In the Illawarra, 19 demountables have been at the same schools for more than 10 years.

The longest-serving demountable has been at Albion Park High School for just over 19 years, while Kiama High School and Dapto Public School have the highest number of older demountables with three apiece.

Mr Park said it was concerning the demountables had not been replaced by permanent structures after being on site for so many years.

"Demountables are meant to be just that in the sense they are not meant to be permanent buildings," the Keira MP said.

"While they are a necessary part of our education system, we don't want to see them on site forever and a day.

"All governments need to make sure they are actively looking at this issue."

He urged the state government to reinstate the demountable replacement program, scrapped in 2012.

A spokeswoman for the Department of Education said demountable classrooms were regularly refurbished and were used to manage changing enrolment patterns and meet needs arising from emergencies such as fire or school maintenance.

She said the demountable replacement program had been scheduled to end in the 2010-11 financial year with no future funding provisions and that demountables continued to be replaced through the capital works program.

"The use of demountables in schools is constantly being reviewed by the department based on a number of factors, such as a school showing a long-term need for additional accommodation due to sustained and stable enrolment growth."