its a policy that as usual appears to have been announced based on no actual research and with no thought given to how it will be perceived. It's almost like David Shearer just woke up one morning and decided "ooh we'd better announce something to get the support of NZ First voters".
When a policy ad leads in with a statement like OVERSEAS SPECULATORS ARE DRIVING UP HOUSE PRICES the first thing you should say is "are they? prove it".
As far as I am aware, most NZ residential property owned by non-residents is purchased by Australians in the process of moving to NZ or by Kiwis living overseas. So, Labour: show me some fucking numbers.