Mr Finlayson said the "on-account" arrangements were not new for iwi in negotiations with the Crown and had been used used by many iwi in the past.
"They must pay the full issue price and any amount drawn down will be deducted from the final settlement."
If yet-to-settle Iwi meet criteria for eligibility, they would be able to buy ordinary shares at the IPO price, on the understanding that the purchase forms part of their overall settlement package.
They would not be eligible for loyalty bonuses, as these will apply only to individual New Zealanders.
A 5 per cent maximum of their likely settlement would be available for Iwi not "local" to any of the power companies' assets.
A 10 per cent limit would apply to Iwi who are local to any of the companies and a 12.5 per cent limit would apply to iwi who are local and had reached an Agreement in Principle with the Crown over their claim, and agreed an amount for settlement.
Mr English said the actual amount to be advanced as on-account payments for shares would depend on the level of take-up by iwi.
Treasury estimated that if all Iwi took up their full entitlement, that would amount to $145 million over the entire Government share offer programme.