The Finance Minister has raised some concerns over NZ First's bid to break up the supermarket duopoly. 

NZ First has announced that if re-elected, it will split Foodstuffs into two nationwide co-operates: one for New World and Four Square, and the other for Pak'nSave. 

Nicola Willis says it's good idea, but it needs to be carefully implemented. 

"That's why I've been taking my time working through proposals, looking very carefully at costs, very carefully at benefits, reviewing that, thinking about implementation - because I agree with Winston that the margins in our supermarket sector are very high."

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