The first of the banks to lodge its disclosure statement for the December quarter, BNZ's mortgage book grew by $227 million to $27.61 billion, based on its loan-to-valuation ratio (LVR) tables.

That followed BNZ's $305 million growth in the September quarter and brought its mortgage book's growth for calendar 2011 to $1.18 billion.

Reserve Bank figures show mortgage lending by registered banks grew $1.28 billion in the three months ended December - interestingly, most of that growth came in the month of October with lending in November and December increasing just $22 million.

If the central bank's figures were an accurate proxy for the figures in banks' disclosure statements, that would mean BNZ accounted for 17.7% of bank mortgage lending in the December quarter. Its market share at September 30 was 16.39%.0

BNZ's figures suggest much of its mortgage book growth has been through relaxed credit criteria - its loans with LVRs above 80% grew by $138 million in the three months ended December, or 60.8% of the book's overall growth.

Loans with LVRs between 80% and 89% rose by $73 million to 6.5% of its total book at December 31 from 6.3% three months earlier while loans with LVRs 90% and above rose by $65 million to 7.2% of the total book from 7.1% three months earlier.

BNZ's net profit for the three months ended December jumped to $289 million from $150 million in the same quarter a year earlier.

Most of that growth came from net unrealised gains on financial instruments, although net interest income rose 7.2% to $358 million and charges against profit for bad loans dropped to just $3 million from $39 million in the year-earlier quarter.