TSB's June quarter disclosure statement shows its mortgage book fell by $3.9 million in the three months to $2.44 billion, the first time it has declined since the June quarter of 2008.

Its mortgages with loan-to-valuation ratios (LVRs) above 90%, the category which includes Welcome Home Loans, shrank by $17.4 million while those with LVRs below 80% grew $10.7 million and those with LVRs between 80% and 90% grew by $9.2 million.

Managing director Kevin Murphy says as well as the government move, the mortgage book continues to be affected by its customers concentrating on paying down debt.

If Reserve Bank figures prove to be an accurate proxy for the figures disclosed in banks' quarterly statements, that meant TSB's share of the mortgage market eased to 1.41% from 1.43% in the three months.

TSB's profit rose 22.4% to $14.4 million compared with the June quarter last year, reflecting an 11.9% rise in net interest income to $27 million and charges against profit for bad debts dropping to $0.4 million from just over $1 million in the year-earlier quarter.

Murphy says the increase in net interest income reflects the trend towards higher-margin floating rate loans which continued through to April this year. As with other banks, Murphy says TSB is now seeing a switch back to fixed-rate mortgages.

"We're beginning to see a little of that. It's a reflection of the continued low interest rate environment."