It might be the country's biggest bank, but ANZ has scored at the bottom of Consumer NZ's latest bank satisfaction ratings.

The advocacy group surveyed 1920 banking customers about 17 service areas.

The Co-Operative Bank was the top performer in the survey, with a score of 77 percent. ANZ was at the bottom, with 57 percent.

The average across all banks was 64 percent.

Chief executive Jon Duffy said it was the fourth year in a row that The Co-Operative Bank had been ranked top. It performed particularly well for digital banking, savings interest rates and advice.

"It's an impressive result, especially considering its market share - less than 1 percent.

"Bigger is not necessarily better. ANZ is New Zealand's most profitable bank, with the biggest market share, but when it comes to customer satisfaction, it finished bottom of our survey."

ANZ performed poorly for interest on savings, fees, responsible lending, advice and overall value for money.

Duffy said it probably came down to a question of how hard banks had to compete. He said ANZ did not have pressure to do better.

"They're sitting on the largest market share, we know … people aren't switching and even when they do switch, what we saw come out of the Commerce Commission market study was they don't tend to switch away, they add a bit of functionality at another banks - so they might add a mortgage at Kiwibank but their savings stay at ANZ. It's not switching like you see in electricity sector."

ANZ has been approached for comment.

He said "doing better" would mean using resources that would reduce profits. "We do see that across the big four - high profits, pretty poor customer service."

He said small banks had to be scrappy. "The little guys who are fighting for your custom are incentivised to offer better service to lure you across. They have to work harder. The big guys are sitting on these incumbent customer bases and they can just cruise."

Duffy said he would hope that banks would look at the results and think about they could do better.

"[ANZ] has got such a large market share, if they were doing better, the banking sector as a whole would be doing better."

The survey also showed that one in five New Zealanders had been victim of a scam involving their bank account or a financial service.

Duffy said while progress had been made to address scams, New Zealand was still lagging other countries.

"Banks have rolled out confirmation of payee and a reimbursement code is on the way, which is a step forward.

"It's not perfect but I acknowledge the efforts being made there but we are a long way behind."

He said New Zealand was also lagging when it came to open banking,

"That's probably because of the structure of the market, the big four don't have to be offering innovative services to turn the very large profits they do. They can get away with not innovating and still make a profit."

ANZ recently posted a profit for the six months to the end of March of $1.27 billion.

Greg Smith, head of retail funds at Devon, said it was a strong result but was driven by gains in economic hedges.

"It is worth noting that impairment experience was low in both Australia and NZ despite economic challenges - people have been getting by in this downturn so there haven't been the credit write-offs perhaps associated with other downturns. Net interest margins have meanwhile contracted in Australia but expanded in New Zealand."