Some banks are advertising one mortgage rate to the public and offering a lower rate through mortgage brokers, with one expert saying that if "you don't ask, you won't get".
It comes as home loan rates have risen and may keep climbing - but house sales are down, meaning banks are writing fewer loans.
Loan Market mortgage adviser Grant Patten joined Breakfast to discuss the situation this morning.
He said banks were "absolutely" acting above board.
"The banks are there to do a job... I think it's great for the consumer, because we're getting competition now," Patten said.
"That competition in mortgage rates is really important so that people actually, in this time when there's a real cost of living squeeze, have the opportunity to shop around."
And a key factor in the move is that the broker share of the market has grown "significantly", he added.
"The broker channel is really important to the banks because people have got less time, and have less ability and less knowledge.
"The one thing to note with all these specials that have come out is, they are for new-to-bank customers generally, or refinancing customers, or top-ups of existing customers only.
"They don't apply to rollovers of existing loans... I think banks are now really trying to fight for that mortgage space."
Ultimately, Patten's advice to customers was "the old adage; you don't ask, you won't get".
"The first step is, phone your bank if you've never been through an adviser, and if you don't get a response within 24 hours, I'd suggest you go to an adviser and get some advice around it.
"Absolutely shop around... if you go through an adviser, our job is to shop around for the best deal and see if it fits you first because you may be wasting your time.
"What I do suggest, act quickly though."
Information on home loan interest rates is available from ASB here, BNZ here, ANZ here, and Westpac here.
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