The group said the Glasgow-based lender, which also owns Yorkshire Bank, saw “negligible” customer compensation costs in the first quarter.
But NAB said there remained a wide range of “uncertain factors” to determine the total costs associated with what it described as conduct-related matters. It did not provide a figure for the increased provision.
NAB’s chief executive, Cameron Clyne, right, said: “Regulators continue to take an active stance in our management of customer claims, including for payment protection insurance and interest rate hedging products.
“There has also been an increase in the level of complaints and settlements relating to some tailored loans.”
Clyne said a full assessment on the level of provisions would be made when NAB finalises its accounts for the six months to the end of March.
In yesterday’s NAB update for the three months to 31 December, it said cash earnings at its UK banking arm improved, driven mainly by lower costs and lower bad debts.
David Thorburn, chief executive of Clydesdale and Yorkshire banks, said he was encouraged by the progress made at the businesses but stressed that more needed to be done.
“We’re looking at all aspects of the service, support and value we provide our customers.”
He said that to underline a commitment to dealing with past conduct issues and learning from mistakes, the UK arm has created a “dedicated customer trust and confidence” function.
“The team will drive a wide-ranging programme of change to embed the culture and processes that will ensure all our products and services deliver fairness for customers now and in the future,” he added.
NAB said its unaudited cash profit rose 7 per cent to A$1.55 billion (£830 million) for the three months to 31 December.
The group said that bad debt charges throughout the bank fell 23 per cent in the quarter.
In its last financial year, the Clydesdale and Yorkshire banks made a profit of £127m, following a loss the previous year of £183m.
But they had to take an extra £130m hit for payment protection insurance (PPI) mis-selling, taking total provision to £350m.
NAB also made a £36m provision in the year for interest rate swaps mis-sold to small businesses, plus £13m for credit card insurance sold through financial services firm CPP Group.