Credit unions and mutual banks lost ground in the retail deposits market in December, as major banks appeared to reassert their market power.Official data published by the Australian Prudential Regulation Authority show that the household deposits pie expanded by over A$6 billion during the month to $880 billion.However, the major banks accounted for more than $5 billion or 81 per cent of the new money flowing into consumer accounts, with National Australia Bank recording the fastest growth rate among the Big Four.The December data might be the first indication that the market share gains posted by small institutions throughout most of last year might not be sustained in 2018.Many mutual deposit takers suffered net outflows of cash during the month including Australian Military Bank (down $7 million), Community CPS (down $29 million) and Defence Bank (down $14 million).While these declines might be attributable to seasonal factors, the major banks enjoyed one of their most successful deposit-raising months in several years.NAB is continuing to build market share among household depositors at a faster rate than its peers.In December it expanded its retail deposits by $1.15 billion - almost 1 per cent - to more than $125 billion.ANZ and Westpac each grew their equivalent deposit bases by more than 0.7 per cent to $119 billion and $206 billion, respectively.Westpac has consistently outgrown Commonwealth Bank in market share terms over the last two years.There is a chance, if this trend persists, that Westpac could overtake CBA - through organic growth alone - as the country's largest retail deposit taker in the next decade.CBA increased its retail deposit base by $1.4 billion or 0.57 per cent to $250 billion in December.After reclaiming market leadership in the business deposits market from CBA in September, NAB appears to be consolidating its position.According to APRA, NAB had $115 billion in deposits from corporate and business customers at the end of December - an increase of almost $1 billion on November.CBA suffered a net outflow of business deposits during the month, with its base sliding more than $1.4 billion to $111 billion.While CBA struggles to grow consistently in the business segment, its market position is coming under threat from Westpac.Westpac boosted its deposits drawn from corporates and small businesses by around $1.5 billion to over $109 billion.ANZ remains the smallest deposit taker of business deposits among its peers.It had a base of almost $93 billion at the end of December, a rise of about $250 million for the month.