"The one thing that's worse than profitable banks is unprofitable banks, so all these countries are finding out what happens when your banks fall over."
Three out of New Zealand's Australian-owned big four banks recently reported record results and between them generated net profits of $3.5 billion for the year, up from $3.2 billion on the previous year.
Mr Hayes told the Wairarapa Times-Age his question was prompted by complaints from constituents that overdraft rates were too high.
"There's reasonable competition. You always have collusion, I'm not accusing banks of that but the accountants I'm talking to, people I respect and respect their judgement, are telling me there's a problem, at least a problem in our part of the world."
Mr Hayes said farmers could be saving $20,000 a year in interest if banks dropped overdraft rates on the million-dollar debts many farmers had.
Federated Farmers Wairarapa president Jamie Falloon could could not be reached for comment.