Australians are failing to stick to a healthy and balanced diet, and junk food is a major culprit.

That's the verdict from the national science agency CSIRO, which on Tuesday dealt Australians a disappointing average diet score of 55 out of 100.

"Discretionary" or junk food was a primary source of the downfall, ranking as the lowest area of diet quality across all ages and sexes with a score of 20 out of 100 when compared against the Australian dietary guidelines.

The major contributors to the pitiful score were alcohol, cakes and biscuits, chocolate and confectionary, and takeaway food, the CSIRO's Healthy Diet Score report found.

On average, people consumed about 28 serves of junk food each week.

People working in the construction and beauty or fashion industries ate the most junk food, and construction workers were among those with the worst diets overall.

Only four in 10 adults ate three or more different vegetables at their main meal, although women ate significantly more vegetables than men, the report found.

Women also had a slightly better overall diet quality than men.

Retirees and people working in the fitness industry were among the country's healthiest eaters, the report found.

The average score of 55 was a stark reminder Australians had to do more to improve their eating habits and reduce the national waistline, CSIRO research scientist and report co-author Gilly Hendrie said.

"Improving our collective score is important to increasing our wellbeing, tackling Australia's obesity crisis, and mitigating lifestyle diseases such as heart disease, Type 2 diabetes and some cancers," Dr Hendrie said.

"It is clear that we have a long way to go."

Dr Hendrie urged Australians to reduce the amount of junk food they ate, increase the amount of healthy foods they ate and eat three or more different types of vegetables with their main meal.

The simple changes would put Australians on the path to a better diet quality score, she suggested.

Australians achieved their best diet quality score in the beverages area with 93 out of 100, as most opted to drink water rather than energy-heavy drinks.

They also predominantly stuck to dietary guidelines when it came to meats and alternatives, gaining a collective score of 78 out of 100.

The Healthy Diet Score report, released on Tuesday, was based on an online survey of more than 235,000 adults between 2015 and 2023.