A 20 per cent tax on sugary drinks is being proposed by Australia's leading health organisations as part of a tough new strategy to tackle obesity, which they now say poses a greater risk to the nation than smoking.

A coalition of 34 high-profile groups including the Obesity Policy Coalition, Cancer Council, Royal Children's Hospital Melbourne and the Stroke Foundation are calling on the federal government to establish obesity prevention as a national priority.

The strategy includes a ban on unhealthy food advertising on free-to-air television during prime time, between 5.30pm and 9.30pm, when they say the greatest numbers of children are watching.

"This is really urgent," said Obesity Policy Coalition executive manager Jane Martin.