The graph below depicts the 'support ratio' for NZ over the next 46 years. The support ratio is the ratio of workers to consumers. The best way to reduce the drop is through greater immigration (the blue line) although at some point immigrants also become consumers and not workers. Increasing fertility takes women out of the workforce so produces more consumers in the short term (the green line). Whichever way you look at it by 2060 the ratio will be probably lower than ever before. And in 1960, when families produced three, four or five children, the consumers were generally less expensive than those of the future - more predominantly the elderly. Though I guess that's a debatable point.