Research conducted by Mozo has identified sustained price leadership as the main driver of impressive deposit gains posted this year by a clutch of regional and customer-owned banks.The latest monthly banking statistics published by the Australian Prudential Regulation Authority show that non-major banks continued to win market share from the majors in retail deposits. In the six months to the end of May, three challenger brands -  AMP Bank, ING Direct and ME - grew their household deposit books faster than all other institutions that held deposits of more than A$1 billion.Since launching its hybrid transaction savings account in January, AMP Bank has grown its deposit book by a whopping 26 per cent to $3.9 billion.In the same period, ME Bank's price-leading bonus saver account has propelled its retail base to $5.8 billion - up 18 per cent.And the popularity of ING Direct's Savings Maximiser account has been so strong that the Dutch-owned bank has unseated Bendigo and Adelaide Bank as the fifth largest deposit taker in the Australian market, with $27 billion.The official data shows that ING has added more than $2.3 billion of deposits since December 2016, which equates to growth over this period of more than nine per cent.A string of customer-owned banks are also increasing deposits well above the system growth rate of three per cent.They include Teachers Mutual Bank, which pushed its retail deposit book in May to above $5 billion for the first time. Teachers Mutual has grown its book by almost eight per cent in the last six months.Queensland-based Heritage Bank is up 6.4 per cent since the start of December and now holds just under $4.8 billion.Mozo's data services director Andrew Duncanson attributed the growth of these five non-major banks to sustained price leadership in term and at call segments of the market."Over time there is a clear correlation between top quartile pricing and gains in market share," he said."You can see that in the one year term deposit market where Teachers Mutual, ME Bank and Heritage are the only banks offering savers rates above 2.75 per cent."While the four major banks now hold 83.95 per cent of consumer deposits - that is down on the 84.01 per cent share they enjoyed in May 2016.Only one of the majors, ANZ (up 4.2 per cent), has grown faster than system in 2017. The country's largest deposit taker, CBA, increased its book by only 2.1 per cent.After almost a decade of better-than-system growth, National Australia Bank appears to have lost momentum.Its deposit base has expanded by only 2.7 per cent this year, probably because its online arm, UBank, is no longer a consistent outright price leader on bonus saver accounts."We've definitely noticed that UBank has changed its position in the marketplace both in terms of pricing and the marketing dollars spent promoting its products," said Duncanson.As NAB weighs its strategic commitment to UBank, Citigroup and Bank of Queensland appear to have even bigger challenges. Although Citi retrieved some momentum in May, its vaults are