The country's four major banks are continuing to lose market share in credit cards as consumers switch to low rate offerings of smaller issuers.The latest analysis of official APRA and Reserve Bank data by MWE Consulting shows that ANZ and Westpac suffered the largest market share losses in the 12 months to the end of April.While Westpac remains the second largest card issuer with A$9.3 billion of credit card balances, its business is also bleeding the most.In April last year Westpac accounted for almost 19 per cent of national credit balances but that has since declined to 18.1 per cent.ANZ's credit card arm is also haemorrhaging, with its market share sliding to 14.7 per cent from 15.3 per cent in the same period.Commonwealth Bank, the largest card issuer in Australia, is also losing ground but at a slower rate than its peers.CBA had $11.1 billion owing on its credit card book at the end of April, giving it 21.5 per cent of the market - down 0.4 per cent on its share of balances a year ago.Total balances on NAB's credit card book also fell 0.4 per cent over the 12-month period to 10.9 per cent.The big winners appear to be Macquarie Bank, Citi, Bendigo and a sweep of mutuals that have sweetened their product offers over the last two years.Citi and Macquarie are also benefitting from a string of white label deals that have boosted the volume of cards they have on issue.The main kicker for Citi in the last 18 months was the acquisition of the issuing rights to the Coles card.Since April 2016, Citi's share of Australian card balances has surged from 8 per cent to 9.3 per cent.If current trends persist over the next three years, Citi is likely to overtake NAB as the fourth largest card issuer in the country.Another trend identified in the MWE report was the sharp decline in the respective market positions of Amex and Diners Club.The respective decisions of the major banks to abandon issuance deals for Amex's companion card have punched a hole in the global card company's local market share.According to the RBA data, Amex accounted for 17.3 per cent of card-based purchases in the 12 months to the end April - down from 18.8 per cent for the same period to the end of April 2017.The market share losses appear to be accelerating with Amex accounting for only 15.3 per cent of card purchases in April - down from 19.3 per cent in 2017.