The swaps allow clients to manage the interest rate exposure on their borrowing and are typically marketed to large corporations and institutions. However, from 2005 banks began marketing them to their rural and commercial clients.
The commission began its probe in August last year and has received 42 complaints since concerns over the way the financial derivatives were sold first aired in the media.
A spokesman for ANZ Bank said the issue related to some rural interest rate swaps mostly sold by the National Bank to large farming enterprises before the global financial crisis when international events led to interest rates suddenly dropping by unprecedented levels.
"Immediately following the global financial crisis we worked with our customers, including many farmers, who found themselves in challenging financial circumstances."
The spokesman said ANZ remained committed to working with its customers and would continue to work with the commission on the historical rural interest rates swap issue.
ASB said it was unable to comment on the commission's claims other than to say it understood that the commission's concerns relate to historic transactions.
Westpac said it would seek details about the nature of the commission's specific concerns.
"It is difficult for Westpac to respond given the generality of the commission's statements and the absence of any specific information from the commission regarding Westpac's activities."
Any proceedings would be vigorously defended.
- additional reporting BusinessDesk