Am I correct in assuming that the franking credits that often come attached to ASX companies dividend payments are no use to an NZ tax resident?
That is correct (https://www.ird.govt.nz/income-tax/income-tax-for-businesses-and-organisations/income-tax-for-companies/imputation-for-companies/imputation-between-australia-and-new-zealand/trans-tasman-imputation-for-new-zealand-shareholders).
To my knowledge they offer shielding from Australian non resident withholding tax (I think it is 15% of the dividend). A fully franked dividend will have no deductions for AUS tax, but the IRD will take the full 33% resident withholding tax. If not fully franked, both AUS and NZ take some tax but the total tax paid will always be 33%. I am not an expert :o but that is my current understanding.