Westpac Bank's mortgage book grew by $518 million to $36.98 billion in the three months ended June.
In the same three months, Westpac chalked up a 38.6% jump in net profit to $122 million.
Most of the bank's growth in mortgages, $515 million, was in the safest part of its book, lending to those with deposits greater than 20%.
Radio New Zealand's business editor said the riskiest part of its book, Westpac's lending to those with less than a 10% deposit, fell by $66 million to $2.85 billion.
However, Westpac's lending to those with deposits of less than 20% but more than 10% rose by $69 million to $5.75 billion.
Nobody from Westpac was available to comment, but its numbers support anecdotal evidence that banks are already tightening their lending to those with skinny deposits ahead of any limits imposed by the Reserve Bank.
Earlier this week, the Reserve Bank said it might limit such riskier mortgage lending by each bank to no more than 15% of total new lending.
Westpac's figures show only 0.6% of its net new lending in the June quarter was in this riskier category, although it still has the highest proportion of total lending in this category of any of the major banks.
