The stranglehold of Australia's four major banks over retail banking in New Zealand is under attack from a leading Kiwi politician who is likely to have a big say in who will form government after the upcoming national election.Winston Peters, a former deputy prime minister and perennial leader of the New Zealand First Party, has warned Westpac and the Kiwi arms of other Australian banks that their domination of consumer banking will be exposed to greater scrutiny in the next parliament.New Zealand's general election is scheduled for 23 September and the structure of the country's banking market has emerged as an issue in public debates in the last week.Peters' New Zealand First party currently holds twelve seats in the national parliament but support for the party is on the rise according to opinion polls.While Peters is sometimes accused of wanting to nationalise parts of the banking system, in an interview with Banking Day he ruled it out."I'm not a communist - I'm a responsible capitalist," he said."But I think the neo-liberal experiment that resulted in all of our banks being sold to foreign banks has failed."Peters is a controversial figure in New Zealand politics but his influence can never be discounted - in the last twenty years he has twice played the role of a parliamentary kingmaker.In 1996 his party formed a coalition government with the National party and nine years later he was appointed foreign minister under a deal struck with the minority Labour Government led by Helen Clark.An average of opinion polls taken in the last two months shows that public support for the two major parties is on the decline, which likely means New Zealand First and the Greens are poised to exert policy influence over the next government.Banking reform has been a big theme in early campaigning by Peters and the Greens, and both are promising a crackdown on the market power of the Australian banking subsidiaries.Peters argues that retail customers in New Zealand are slugged with higher fees and charges than Australian customers but that recent National Party governments led by John Key and Bill English have been too feeble to take action."They've exhibited all the characteristics of puppets," he said."The big four Aussie banks are ripping our country off."New Zealand First is now campaigning on a promise to launch a wide-ranging inquiry into the banking system and to explore ways "we can grow our own New Zealand banking sector".Peters raised the stakes further over the weekend in a speech to party members in Auckland, promising to scrap a transaction banking contract between the New Zealand government and Westpac.The deal, negotiated in 2015, has six years to run but Peters says he will use his leverage in the next parliament to transfer the lucrative arrangement to the state-owned Kiwibank."Westpac is clipping the ticket on every government transaction there is and we think that is ridiculous," he told Banking Day."From the point of view of the national economy it makes no sense to have our banking