The legislation required lenders to scrutinise customers' expenses more closely when assessing loan applications, along with other measures designed to protect vulnerable consumers.
"We knew some customers wouldn't be happy about this greater level of scrutiny - or the resulting longer processing times," Sladden said.
"We issued a guide in October to help customers prepare for the changes, but many would still have been caught by surprise."
In total, banks received 24,206 complaints during the quarter, up 3 per cent on the previous quarter.
Of these, 12 per cent were about what customers considered to be a bank's failure to follow through on an agreed action, particularly in lending-related matters.
Sladden said the tightening of loan-to-value ratios and an increase in interest rates during the quarter contributed to the spike in lending-related complaints.
In November, the Reserve Bank halved the amount of low-equity lending banks were able to provide, leading some banks to withdraw or modify pre-approvals issued to borrowers with deposits of less than 20 per cent.
The scheme also received more lending-related complaints between October and December, with a doubling of concerns about delays and banks not acting as promised, as well as a significant increase in concerns about unfair fees and rates.
On a more positive note, she said there had been a drop in the number of complaints related to financial hardship, which indicated the banks were managing those more effectively.
New Zealand Bankers' Association chief executive Roger Beaumont said it was no coincidence that lending related complaints had risen at the same time the government introduced new lending rules that made it harder to get a loan.
"The new rules mean it takes longer to get a loan because banks need to collect and verify more detailed information from customers.
"It's also harder for banks to approve loans because of the surpluses customers need to have, on top of banks' existing responsible lending practices such factoring in potential interest rates rises."
Beaumont said it was not surprised the government's new rules were making some customers unhappy, and front line workers were bearing the brunt of that.
"It's also not surprising they're complaining to the Banking Ombudsman about it, even though we're not responsible for the rule change."
The Government has recently announced tweaks to the law change which are under consultation and are set to come into force in early June.
But Beaumont said it was not convinced the tweaks would do enough to make a difference for customers.
"More could be done to reduce the impact on most consumers while maintaining protections for vulnerable consumers. We'd like to see the new rules work in a way that doesn't restrict access to responsible lending for consumers who can afford it, while ensuring vulnerable consumers are protected from high-cost credit that may not suit their circumstances."
