Kiwibank is renewing its focus on cost control after reporting only a slight rise in annual profit despite a 10.6 per cent rise in lending and a rise in net interest margins.Twelve years on from its launch as a market share driven competitor for New Zealand's Australian-owned Big Four banks, Kiwibank chief executive Paul Brock said the state-owned bank was looking to improve efficiency and reduce costs.This followed a three per cent rise in net profit to NZ$100 million, driven by a rise in its net interest margin to 1.86 per cent for the year from 1.81 per cent the previous year that helped lift net interest income by 6.2 per cent to NZ$293 million. Cost growth of 13.2 per cent to NZ$344 million increased Kiwibank's cost to income ratio to 71.8 per cent from 68.1 per cent a year ago.  Profit growth was also supported by a drop in bad debt expenses to NZ$59 million from NZ$72 million after continued strong economic growth and rising house prices in the fastest growing markets of Auckland and Christchurch. Brock said investments in a new core banking system, new stores and digital initiatives had increased costs."This issue is being addressed with an internal restructure designed to improve both customer service and productivity and reduce costs," he said.Brock said Kiwibank had reached an important stage in its growth, having built up customer numbers to 860,000. It increased its share of the market for primary bank accounts to 11 per cent from 10.3 per cent over the year and had won seven per cent of the residential mortgage market over 12 years. He said the bank's challenge was to maintain profitable growth. Brock told a news conference the bank expected to begin paying dividends to its parent within three years.