Kids on average see three unhealthy food ads for every hour they watch TV during peak viewing times.

The food industry has rejected claims it uses a "loophole" to get around self-imposed restrictions on junk food advertisements aimed at children.

Analysis conducted by Cancer Council NSW and the University of Sydney found children are being exposed to too many unhealthy food ads when they watch TV.

It also found there'd been no reduction in junk food TV advertising during children's peak viewing times since 2011.

The study, published in the Journal of Public Health, found that aon average kids view three unhealthy food ads during every hour they watch TV.

That's usually between 6am to 9am and 4pm to 9pm on weekdays and 6am to noon and 4pm to 9pm on weekends.

The researchers analysed ads broadcast during these times on the three major free-to-air commercial television channels in Sydney over a four-day period in 2015.

Nearly half - or 44 per cent - of food ads were for unhealthy foods.

One in five were for fast food, mostly from McDonald's (47 per cent).

Chocolate and confectionary and sugary drinks were also frequently advertised.

"These are low nutrient, high fat, high sugar foods," Cancer Council NSW nutrition programs manager Wendy Watson said on Wednesday.

In response to rising childhood obesity rates, the Australian Food & Grocery Council undertook self-regulation in 2009.

The plan was to control the promotion of unhealthy foods to children under the council's Responsible Children's Marketing Initiative.

But Ms Watson says it's not working because because of a loophole.

Often children are also watching TV during times that aren't defined as kids programs, like reality TV and sports.

"The percentage of children in the audience might be small but these are the top rating shows, so they have a huge audience and a huge children's audience as well," Ms Watson said.

"That's the loophole that is being exploited and that we are trying to highlight."

But the industry rejects the research.

"This report measures all food advertising, which is almost entirely directed at adults, not at children," a council spokesperson said in a statement.

"The codes deliberately target programs when children are likely to be viewing TV on their own, without parental supervision, which is the rationale for removing promotion of non-core foods during these programs."

Cancer Council NSW wants on the federal government to enforce regulations to reduce the influence of junk food sellers.

"Junk food marketers are influencing what children eat and they are more likely to put on weight then," she said.