The latest RBNZ review on monetary policy and the level of the Official Cash Rate brought this note from them:
The Committee expects that higher wholesale interest rates will be reflected in higher retail interest rates, particularly deposit rates, as banks compete for funding.
It seems a fair comment to make.
The margin between the average retail term deposit rates and wholesale rates has never been lower over the past 15 years.
While there isn't any financial law that says term deposit rate offers need to exceed the cost of wholesale funding, over this same period it has for most of the time. That history shows it has exceeded it by about +1% on average for both the six month TD offers (actual = +0.937%) and the one year TD offers (actual = +1.07%).

At present, average six month term deposit offers are -0.8% below the cost of six month wholesale money, and the average one year term deposit offers are -0.4% below the cost of one year wholesale money.
If both were +1% above the wholesale benchmarks, six month term deposit offers would currently be offering 4.40% pa and one year term deposit offers would be offering 5.20% pa. The highest offer from any main bank is now a full 1% pa lower than that.
Savers are motivated by higher term deposit interest rates. As they have risen recently, there has been a sharp shift from low or non-earning bank accounts back to term deposit accounts. Savers will shift when incentivised by a higher interest rate.
On the other hand, banks are commercially motivated and incentivised to keep offers low. Only competitive pressure will raise them. Some of that "competitive pressure" comes from the wholesale cost of money, some comes from the opportunities to lend, and some comes from savers who are prepared to shift institutions for a better rate. It is this last motivating factor that is the weakest. Banks rely of their 'replicating portfolio' - savers too lazy to make the effort to switch.
It is not as though there aren't higher offers from banks - they are just from challenger banks rather than "main banks". Savers have always seemed reluctant to support challenger banks in sufficient volume to cause main banks to change their behaviour. To be fair, because all challenger banks are small, and between them have less than a 10% market share, their capacity to take a volume of fund flows sufficient to affect the main banks isn't great. The main banks can rely of this incapacity.
Despite all that, the offers on the table at present are unusually low. The RBNZ is right to call them out. If the main banks can ignore the pressure from the challenger banks, will they also ignore the signal from the RBNZ?

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