The big four bank moved to new pricing terms last week. The new rules mean that pricing offers made by the lender can be changed if rates move during the five day validity period.
The change has already caught out some clients who have agreed mortgage rates, only to see them changed by the bank to higher pricing.
BNZ, like its rivals, has hiked rates over the past few weeks, meaning several advisers and clients have been impacted by the new model.
Fixed rates have increased across the board ahead of the Reserve Bank's expected OCR hike early next month.
One adviser told TMM Online that they had received pricing from BNZ for a restructure on a Friday, and were given a five day validity period, only to be told they would be moved to a higher rate on Monday.
"It essentially means that any conversations that we have with the client about how to structure lending under the new responsible lending practices is immediately invalidated because the rates increased by 30 points, even though we had a five day offer in place.
"The offer isn't worth the paper it's printed on. So the question is, is this something that is acceptable practice by a lender?"
The new pricing conditions have drawn a furious backlash on social media, with several brokers hitting out at the bank on Facebook.
Brokers called the policy "false or misleading", "shocking", and "deceptive", while one accused BNZ of "moving the goalposts".
Some advisers are believed to have complained to the Banking Ombudsman about the new policy.
Advisers told TMM Online that the changes have led to additional work as brokers are forced to re-engage with clients to discuss the higher rates.
A BNZ spokesman said the policy change was made to bring brokers in line with other BNZ customers.
"Last week we brought broker channel pricing conditions into line with those in the rest of our channels. This provides consistency for customers no matter the channel they choose to do business with us," the spokesman said.
"Pricing is subject to change, but if a customer wishes to lock in a particular rate and term they should consider a Ratelock."
The spokesman added: "New customers need to be unconditionally credit approved, but existing customers can get one [a Ratelock] as long as it’s within 60 days of their current term maturing."
Advisers dismissed the comments and said BNZ was putting advisers under pressure to lock in rates straight away, giving them little time to discuss the deal with clients.
"They're basically saying, we'll give you this rates offer, but if you don't take it straight away, there's a likely chance that it's won't be valid if you don't come back within five days."
"They could give you an offer on a Monday morning, and if the rates change on Monday afternoon, it's gone. So, it gives us no time to discuss anything with the clients."