ANZ Banking Group’s business customers are the banking sector’s most dissatisfied, a survey says.

The latest DBM Business Financial Services Monitor shows ANZ’s customer satisfaction levels in May were the lowest among the big four Australian retail banks in all categories surveyed – overall, micro business, small business, medium business and large business.

For the overall result, Westpac and Commonwealth Bank (CBA) received the highest satisfaction ratings in May, with a score of 7.3 out of 10, while National Australia Bank (NAB) was slightly behind at 7.0.

ANZ came in fourth with a score of 6.8 and had its two worst results in both the micro business (turnover of less than $1 million) and small business (turnover between $1m and $5m) categories, where the bank scored 6.7 out of 10.

DBM managing director Dhruba Gupta said ANZ’s score in the small business category was the lowest since NAB’s 6.7 in February 2011.

“Satisfaction with ANZ has been declining slowly every month since December 2011,” Mr Gupta said in a statement.

“It is now some way adrift of its major competitors which are all relatively steady in this market.”

CBA topped the small, medium business (turnover $5m to $40m) and large business (turnover more than $40m) categories, while CBA and Westpac had the highest satisfaction in the micro business category.

The survey comprised responses from 7,340 decision makers in business banking.