New Zealand Prime Minister John Key will again push this week for Australia to re-open talks on the Trans-Tasman streaming of franking credits, a practice that would benefit Australian shareholders in New Zealand banks most of all.However, it would also be very costly for the Australian Treasury, which has much more to lose from the streaming of franking credits than the New Zealand Treasury. Currently, dividends to Australian bank shareholders from New Zealand earnings are taxed in both New Zealand and Australia. Streaming would mean they were taxed just once, in the country where the profits were generated.Key has foreshadowed his plans to raise the issue when he meets new Australian Prime Minister Tony Abbott for the first time on Wednesday."New Zealand continues to be a strong advocate for the streaming of franking credits," he said.He acknowledged that getting the Australian Government to change its mind would be difficult, but New Zealand would continue to make the case."The last time I looked it was seven times more expensive for the Australian Treasury than the New Zealand Treasury," Key said.Australian investors have a lot more invested in New Zealand than vice versa, particularly given the size of the Big Four Australian-owned banks in New Zealand and the two big newspaper groups, APN and Fairfax New Zealand. Fletcher Building is the top New Zealand-owned company with Australian earnings.However, Key pointed to work by the New Zealand Institute of Economic Research, prepared in August last year for the ANZ Leadership Forum. He said this showed streaming would eventually cost both governments little because of the downstream tax benefits of increased re-investment of dividends once the current distortions were removed.The NZIER report said mutual recognition of franking credits on dividends in Australia and New Zealand would increase Trans-Tasman GDP by NZ$5.3 billion in net present value terms by 2030. It said that about NZ$7.4 billion of dividends are being taxed twice under current rules, meaning Australian shareholders face an effective tax rate of 60 per cent on their New Zealand earnings, while New Zealand shareholders face an effective tax rate of 53 per cent on their Australian earnings."So, we'll be making that case," said Key. "It matters a lot for New Zealand because the bulk of the investor base sits in Australia and the impact of not having streaming of imputation credits is [that] it puts pressure on New Zealand head offices to relocate across the Tasman."Key said he had raised the issue with Abbott when he was previously in government, under the then Australian Treasurer Peter Costello, and Key was New Zealand's Opposition Leader and Finance spokesman. However, Key was sanguine about getting a result, given previous rejections by Australian treasurers."I wouldn't expect deliverables from the meeting," he said.