Massey University banking expert David Tripe said that in 2015 the banks weren't enjoying the same bottom-line benefits from the write-backs of bad and doubtful debt provisions they reported last year.
Meanwhile, net interest income growth had been slowing in the local banking sector.
"That's an effect of competition," Tripe said.
Westpac New Zealand is the last of the big four to report its annual financial result.
At the group level, the four Australian banks' combined profit came to A$30.7 billion (NZ$32.4 billion).
Westpac's $916 million in New Zealand cash earnings was a 6 per cent increase on the previous year.
The Auckland-based bank said impairment charges rose to $47 million from a low charge of $26 million in 2014, which was supported by write-backs.
Net interest income rose 8 per cent to $1.7 billion, while expenses rose 6 per cent as a result of increased investment spending and salary increases.
Westpac New Zealand's mortgage lending rose 5.8 per cent to $41.9 billion.
Business lending lifted 9 per cent to $25.1 billion, while deposits rose 5 per cent to $51.9 billion.
Westpac Banking Group, the New Zealand division's Australian parent, reported a 3 per cent increase in annual cash earnings to A$7.8 billion.
Westpac's result comes after ANZ New Zealand and BNZ reported full-year cash profits of $1.7 billion and $966 million, respectively, last week.
ASB reported a $846 million annual cash profit in August.
See Westpac's full financial results here:
