But he said the law around local body electoral finances "may be very loose".
"The law literally is an ass in this particular case ... anyone can drive a bus through [it]." Mr Key said the Local Electoral Act may need reforming, "and that's something we'll consider in due course".
Changes may be possible before next year's local authority elections, depending on the Government's legislative workload.
Local Government Minister David Carter said he'd had preliminary discussions with officials about aligning the Local Electoral Act with legislation that applies to candidates standing for Parliament, "particularly in regard to anonymous donations".
Labour leader David Shearer said his party was prepared to work with National to improve transparency around donations to local government candidates. 'It's something we've talked about for a long time and I think there's a possibility of sitting down and making sure people have confidence in our electoral laws."
Mr Banks said he would "absolutely" support any move to tighten up the Local Electoral Act.