The lawyers behind a class action lawsuit against New Zealand's banks over dishonour fees say their campaign to sign up over 100,000 complainants is gaining traction, with 16,000 people signing up in less than two days.Independent New Zealand lawyer Andrew Hooker, Slater and Gordon, and Litigation Lending Services launched the signature-gathering exercise on Monday, saying bank customers were being illegally penalised when charged around NZ$15 for going into overdraft, bouncing a cheque or being late with a credit card payment.The group, which calls itself Fair Play on Fees, has stepped up the pressure by saying New Zealand customers of the Big Four Australian-owned banks pay substantially more than the Australian customers of the same banks.Hooker said the cost of bouncing a cheque in New Zealand with ANZ, Commonwealth Bank's ASB or Westpac was, on average, double the amount charged by the banks' parents in Australia. BNZ and parent National Australia Bank do not charge such cheque bouncing fees, but BNZ customers are charged NZ$15 for late credit card payments, about double the amount charged by NAB in Australia.A check by Banking Day of the respective websites confirmed fees for dishonoured cheques and late credit card payments were 40 to 60 per cent higher for the New Zealand customers of Australian banks. "The banks have been happy to charge Kiwis so far. There's no stopping them charging these fees in future if something isn't done," Hooker said."What we saw in Australia is that when everyone stands up to the banks, things change," he said.The New Zealand Bankers Association declined to comment.