Optometry Australia says small children should not watch films and TV in 3D, because it could make the under sixes feel headachy, tired and nauseous, and how it could impact their eyes in the long term was still unknown.

Senior resident optometrist Luke Arundel said the special glasses used to watch screens in 3D got the right eye seeing one image and the left eye to seeing something a bit different.

When the brain fused the two images together the result appeared three dimensional, but some young children found this process made them tired and uncomfortable because it put a lot of strain on their visual systems.

It could be because their eyes were still developing, or it could be due to an undiagnosed eye condition, Mr Arundel said, so parents should take their child to see an eye care professional if he or she had experienced any distress.

But the long term impacts of the exposure to technology, if any, on children's eyes was not known because its widespread use was relatively new, Mr Arundel said.

The brain was learning to see for the first decade of life, so as a safety precaution little children should not be allowed to watch 3D television and films, he said.

Mr Arundel said children aged seven to 13 should be allowed only moderate use, a 3D movie a day was fine but hours in front of a 3D television was not.

"A lot of the time [research is] a step or two behind industry," he said.

IMAX Melbourne Museum manager Richard Morrison said the popular cinema allowed children younger than six to watch films in 3D.

He said in the short term IMAX would not be warning parents about taking their pre-school aged children into 3D films.

"We take occupational health and safety very seriously, and our approach is informed by the relevant government authorities and codes of practice," he said.

Optometry Australia said Australians' eyesight was deteriorating because many people were spending too much time indoors and not exercising their eyes enough.

Children using a tablet, smart phone or computer should be in a comfortably lit room, and should take a break of at least five minutes every hour, he said.

They should also play outside for at least a couple of hours everyday.

"Don't just put them in the lounge room for the school holidays, balance screen time with green time," he said.