Despite advances in classroom technology, the weight of school backpacks continues to grow, with experts advising parents to look for physical signs that it's affecting their children.

Key points: If a child is bending at the waist to carry their bag, it is too heavy

If a child is bending at the waist to carry their bag, it is too heavy Experts urge parents to look inside backpacks to see what is being carried

Experts urge parents to look inside backpacks to see what is being carried Backpacks should sit close to the student's body above the backside

Audrey Connors, 12, said the size of her school bag made it difficult to catch public transport.

"My bag weighs 8.8 kilograms and I weigh 29 kilograms," the Brisbane student said.

"It's really annoying, as I catch the bus home and I walk home — with it being half my size, after a while my shoulders get strained.

"There is still quite a bit of book work to do [for homework] and filling out worksheets, and I have a different laptop case to carry too."

Parents shared their concern about the weight of their children's bags on ABC Brisbane's Facebook page:

"I weighed my daughter's school bag and it was over a third of her body weight. When I brought it to the attention of the school, with suggestions as to how we could change the situation, nothing happened." — Fiona Cameron

"I think all schools should have lockers [as] preventative measures for healthy backs." — Mandy Potter

"We use roller-bags and it is great. I refuse to get them school bags with no wheels because they are too tiny to carry those bags." — Kiru Kan

Audrey's school bag weighs nearly a third of her body weight. ( Supplied: Matt Connors )

How heavy is too heavy?

Nicole Haynes, chair of the Australian Physiotherapy Association's paediatric physiotherapy group, said parents should look for physical signs that it's affecting their children.

"From a physical perspective, if they're having to really lean forward to hold their backpack and bend at the waist, then that's the biggest physical sign you can see," she said.

"We want to make sure kids are independent with carrying their backpacks and encouraging them to be active at the same time."

She said in early primary school years, students often had bags that were too big for them.

"Usually the set school backpacks are only in certain sizes and are often not small enough," Ms Haynes said.

"If you can resist until grade 2 to get the school bag, then students can use an early-year bag that doesn't hang below their bottom.

"The bag's straps should be firm around their shoulders and the backpack needs to be close to the body as well — that's really important."

Many students also have to carry a laptop bag or case along with their backpack. ( ABC News )

Chiropractor Joshua Nelson said questions about the ideal backpack weight were something he heard often from parents.

"There's been a general rule of thumb that it should be 10 per cent of the weight [of the child]," he told ABC Radio Brisbane's Rebecca Levingston.

"For example, if you weighed 40kg, it shouldn't weigh more than 4kg."

Ms Haynes said backpack weight could also vary depending on the physical capacity of the child.

"One person could be strong, yet the other person may not have that physical strength ... it's important for parents to be aware and to look at what they're packing," she said.

One strap or two?

Ms Haynes said studies showed it made no difference if the bag was carried using one strap or two.

"Often children who carry their bag on one shoulder move it from one to the other," she said.

"We don't get too hung up on it as long as the weight of it is controlled to some extent."

Dr Nelson added that parents should check that the bag was not bouncing off the child's buttocks.

"It's good to use the other straps like the chest strap and the waist strap as well to keep the bag secure," he said.

Look inside the bag

Ms Haynes urged parents to look at what items their child was carrying, with excess books and heavy water bottles easily weighing them down.

"For example, the stainless-steel lunchboxes can be really heavy and create quite a weight, so finding a lightweight lunchbox can reduce that," she said.

"Also, a full water bottle can weigh nearly a kilogram in their bag along with extra things like library books.

"You can let them carry their bag, but you could carry their library books — this helps lighten the load but keeps their independence."