That $500 million figure was contained in advice from Treasury to Cabinet last year on the cost of "incentives to encourage domestic participation" in the partial privatisation programme.
Finance Minister Bill English said Treasury couldn't have estimated the cost of the Government's loyalty share bonus scheme "because it hasn't been decided yet". He said he didn't "have a number in my head" about the cost at this point.
"We will decide on the nature of the scheme, and there will be some cost associated with that. But bear in mind, when people are buying these shares, they will know there's a loyalty bonus and that may mean they'll pay a bit more for these shares so that they can get the bonus."
Mr Key suggested the cost of the scheme may be around $60 million to $80 million across the whole programme. Later he said he'd seen reports from sharebrokers "saying that far from being a cost, actually there may be a gain to the Crown from doing this because it encourages others to potentially pay more, namely institutions".
But Labour leader David Shearer said Mr Key was "all over the show".
"Firstly he said the loyalty scheme could cost less than half of the $1.3 billion contained in Treasury papers, then he agreed $360 million was a possible number. This morning, he claimed it could be between $60-80 million and this afternoon in Parliament he said that it might come at no cost. What is the actual figure?
"And why won't the Prime Minister just be honest with Kiwis?"