The adviser association's board sent a fierce email to members last week, in which it said it had been made aware of the "unsavoury practises" of one major bank.
It is believed that some lenders are prioritising mortgage applications received by their own staff, over those from brokers.
That is particularly the case for lending over 80%.
Banks are tightening much of their lending amid concerns about the property price cycle and a clampdown on credit by their Australian parents.
TMM Online approached ASB, ANZ, BNZ and Westpac for a response.
Westpac, BNZ and ASB offered no comment.
At ANZ, spokesman Stefan Herrick said the bank had made no change to the way it worked with advisers.
"We enjoy a good relationship with advisers and hope this will continue," he said.
He would not confirm a rumour that ANZ had pulled its sponsorship from the upcoming IFA/PAA joint conference.
"Our sponsorship arrangements are confidential, so I can't discuss them," he said.
