A George Institute for Global Health study found only about 4350 of 15,770 eligible products on supermarket shelves in 2017 featured a health star rating.

Food multinationals General Mills, PepsiCo and Mondelez, which sell household brands such as Latina, Oreo and Doritos, have opted out of the system altogether.

"It's been four years since it started and there should be a higher impact," lead author Alexandra Jones said. "It's possible for companies to change labels quickly, and we've seen that with the country of origin labelling."

Despite the lack of disclosure, researchers were able to calculate each company's average health star rating and found Mondelez scored a derisory 1.3 stars, General Mills 2.9 stars and PepsiCo 3.1. Mondelez, which owns Cadbury chocolate, Oreo cookies and Ritz biscuits, got the low score based on its 304 products.

The study found some companies were using health star ratings as a marketing tool, slapping them on healthier products and keeping them off junk food, potentially preventing customers from comparing like for like.

Overall, products carrying health stars scored an average rating of 3.4, while products with no stars got 2.7.

For example, while Sanitarium can be applauded for placing a health star rating on 84 per cent of its products, it has chosen to display five stars on its Natural Peanut Butter but no stars on its Marmite (estimated to be 1.5 stars).

Nestle has stamped four stars on Milo snack bars but none on Dunkaroos (1.5 stars). Kellogg's has placed five stars on All-Bran but none on LCMs Split Stix (0.5 star).

"I understand why it began as a voluntary system – it was a pragmatic decision," Ms Jones, who will be attending a government advisory committee meeting on Thursday, said.

"But with the five-year review, we can iron out these issues and the main point is the system must become mandatory if it's going to deliver the maximum public health impact."

Co-author Bruce Neal raised concerns about use of the permitted "energy icon" variant of the label. It should be used only on small packages, but it is being widely used on sugary drinks and confectionery.

“There’s no evidence the energy icon [helps customers] make better choices. They're gaming the system," he said.

"Government needs to close the loophole. Food companies that don’t put health star ratings on their labels, or hide behind the energy icon, are on pretty shaky ground."

On a positive note, the study found Coles and Woolworths were displaying stars on the majority of their products, whether they rated high or low.

Mondelez said the system was voluntary and the government committee was aware of its decision not to take part.

"We are committed to providing our consumers with the tools to make more mindful choices by providing practical on-pack information, such as suggested serving sizes ... and daily energy intake icon," its spokesman said.

The Australian Food and Grocery Council disputed the figures, saying another study showed 10,300 products – or 65 per cent of eligible products – carried a rating.

“The AFGC supports the ongoing implementation of the HSR and supports the review, which will be informed by more up-to-date data," a spokesman said.

General Mills said: "We provide factual icons on the front of pack of all products, stating key nutrition information of energy, fat, saturated fat, sugars and sodium per serving with the daily intake percentage for each."

PepsiCo said its snacks would soon show health stars, which would complement other nutrition information on packaging.

Fairfax Media understands the government will close the "Milo loophole", which means ratings can only be used on products "as sold", with exemptions for rehydration, dilution and draining of water.

Regional Health Minister Bridget McKenzie said companies have been "very engaged" with the government's focus on healthier eating and the findings of the five-year review will be delivered in 2019.

smh.com.au