National Australia Bank has abandoned a pledge to borrowers it would only increase monthly mortgage repayments around the 12-month “anniversary” of variable rate home loans, after rapid-fire interest rate rises caught the bank off guard.

NAB customer Liam Garrety and his wife took out a variable rate mortgage of more than $1 million with NAB in July last year, as part of a refinancing for a new family home in regional NSW and for an existing investment property.