Disposable nappies make up more than 1 in every 4.4 litres of litter tossed in public places across the ACT, according to the Keep Australia Beautiful National Litter Index interim report.

Keep Australia Beautiful chief executive officer Peter McLean said Canberra mums and dads were driving a smelly litter trend that would take hundreds of years to biodegrade.

"It's a shocking example for parents to be setting their children. Presumably, many soiled nappies are thrown out of cars because, like cigarette butts, they smell," Mr McLean said.

"Apart from being unhygienic, it is bad for the environment with some disposable nappies taking up to 500 years to biodegrade.

"Parents are not considering the fact that it will be their own children who will have to clean up environmental damage caused by dirty nappies left to rot along roadsides and in other public places."

Disposable nappies accounted for nearly a fifth of the miscellaneous litter surveyed across Australia in November as part of the report.

They were the third biggest problem after construction materials and tyres, the report revealed.

Mr McLean said while the number of dumped nappies was not as high as other litter items, such as cigarette butts and fast food packaging, the soiled items were taking up increasing landfill space.

In Australia about 5.6 million disposable nappies are used each day, while 2 billion used nappies go into landfill each year.

"Disposable nappies are now so absorbent that even small varieties swell to become relatively large and heavy," he said.

"They take up more room in landfill which is why they are rapidly becoming an emerging litter trend when it comes to volume of waste."

Mr McLean urged parents to dispose of nappies properly regardless of the smell.

"We can only speculate that the nappies are discarded because they're offensive, but surely it's not too much to ask parents to travel with spare plastic bags which they can use to store the soiled nappy until they find a bin," he said.