That means a first-born daughter of William and Kate, the Duke and Duchess of Cambridge, could accede to the throne even if she was born before the law changes.
"If the birds and bees were to deliver that blessing to the Duke and Duchess of Cambridge, and indeed the nation, then that little girl would be covered by the provisions of these changes of the rules of succession because they operate from the time of the declaration of the Commonwealth summit last October," Mr Clegg was reported as saying in the Daily Mail.
Mr Key today said the move was a positive one.
"While tradition is important, I think this is an instance where it is important to move with the times.
"New Zealand is a place where we judge people on their ability and values, not their gender and I am pleased we are part of the process that will result in this positive change."
Monarchy New Zealand spokesman Sean Palmer said making the rule change retrospective to the Commonwealth summit had provided clarity.
"This does a lot to demonstrate the equality of a head of state who is chosen effectively randomly just by the 'accident of birth', as it has been called in the past," he said.
Republican Movement spokesman Lewis Holden said no one could argue with getting rid of "a sexist rule" but the monarchy was irrelevant to New Zealand.
"I think what New Zealanders have to ask themselves is why on earth do we have anything to do with it anyway," he said.
"This isn't our monarchy, this is someone else's monarchy. The fact that they are New Zealand's head of state is really an irrelevance to most people on the street."
