National Party leader John Key says his party would legislate to create a cap on the shares held in any state assets.
The party plans, if re-elected, to sell up to 49% of state-owned energy companies Genesis Energy, Meridian Energy, Mighty River Power and Solid Energy and reduce the Crown's shareholding in Air New Zealand.
Mr Key said on Wednesday that 85-90% of the assets would be controlled by the Crown and mum-and-dad investors.
He said National would pass a law stating that no individual or company could own more than a 10% share.
"There's historical precedent there - Telecom had a cap - it's just a matter of passing legislation. We'd pass it."
Meanwhile, the Green Party has written to the Ombdusman, asking the office to reconsider its decision not to release Treasury documents on National's asset sales programme.
It says the information is being suppressed just days out from the election.
Asset sales good for country, says Key
Mr Key earlier told Radio New Zealand's Nine to Noon programme that under his party's proposal the Crown will have majority control, the companies will perform better and New Zealanders will have something to invest in.
He was asked whether the policy could be up for negotiation in any post-election talks.
"I don't think so, and the reason for that is, I want to go to the country and I want to govern over the next three years with stable government that doesn't load this country up with debt."
Mr Key says the sale of assets has to be conducted effectively so it has public support.
He again ruled out putting any more state-owned companies up for sale in the next Parliamentary term.
Labour has campaigned strongly against any asset sales but Mr Key says it is trying to invoke fear and misinformation.
National hopes to raise between $5 - $7 billion from the partial asset sales though Mr Key acknowledges the anticipated return is uncertain.