The Commerce Commission has finally launched its investigation into supermarket practices across the country after Consumer NZ raised several red flags about its approaches to pricing and promotions.

The commission, a competition watchdog, said it was investigating Woolworths, Foodstuffs North Island and Foodstuffs South Island after Consumer NZ lodged its complaint back in August.

As explained by Consumer NZ's chief executive John Duffy, there were a range of issues that had been brought to the attention of his agency.

"They range from the very simple that people can get their heads around easily - like where the price of something on the shelf is different to what you charge on the till," he told The Mike Hosking Breakfast with Tim Dower.

"So we're worried New Zealanders are being regularly overcharged by supermarkets because the pricing systems they've got in place aren't robust enough to ensure what's on the shelf isn't what's on the till."

However, Duffy said he was also aware of more complex claims such as issues around multi-buys. This involves two-for-one offers where customers would spend more on the two than buying each product separately.

Also concerning Consumer NZ were more general claims about how supermarkets were going about promoting discounts.

"If you're a member of that supermarket's loyalty scheme, there are things that are discounts for that day but there's also a kind of nebulous representation of "everyday low price" - for example, is that item actually on special or is the supermarket using it as it's a good price or a discounted price?"

Duffy said this became "very, very confusing" for the consumers.

If any investigated supermarkets are found guilty of poor promotional and pricing practise, they would be subject to penalties laid out in the Fair Trade Agreement. However, Consumer NZ doesn't believe the penalties are much of a deterrent anyway.

According to Duffy, the last supermarket prosecuted for this type of thing was Pak'nSave in Māngere.

"This was a few years ago, they received a $78,000 fine. And this was them getting caught red-handed with shelf pricing that was different to the till pricing," he said.

"$78,000 is probably what they make in only a few hours' worth of turnover for a business that large. So, we don't think the fines in the act, at least the way the courts impose them, are big enough to deter business of this scale of Foodstuffs."

Duffy said he was thankful that the Commerce Commission had been willing in the first place to look into the matter, considering the amount of work already keeping the commission busy.

However, Consumer NZ would have wanted the commission to jump on the issue sooner, Duffy said, given the problems were flagged back in August.

"I mean, New Zealanders are already struggling with the price of groceries - overlay this leading interceptive pricing and discounting activity on that and you've got consumers doing it tough," he said.

"But we also appreciate that the [commission] is very busy, they have a lot on their plate, they've got many investigations on the go so we're happy they got to it now."

Duffy was also asked about the Government's Grocery Commissioner, Pierre van Heerden, who was appointed towards the end of the tenure of the previous Labour Government to "hold supermarkets to account".

The Consumer NZ executive said that while the commissioner didn't handle Fair Trade Agreement disputes, he admitted it was probably still too early to tell whether the role had made much of a difference.

"The clock is ticking and we'd be hoping to see an initial report out of the commissioner pretty soon," said Duffy.

"[It would] give us the evidence base to start thinking about what needs to be done to improve the situation in the supermarket space, because it's really clear that almost two years on from the [commission's] market study and implementation of the whole new regime that includes the commissioner, that not a lot has changed for consumers."

Duffy said shoppers still faced high prices and the sector still had muted supermarket competition.

"Something does need to change but we do need the analysis to make good decisions and get them underway."