Health insurer NIB says it is being unfairly penalised for its success in attracting younger members and has called for urgent reform of the "risk equalisation" rules, after it downgraded its profit guidance by $30 million.

In a market announcement on Monday, the group revealed its underlying operating profit was now likely to be at least $170 million, down from the original forecast of $200 million. The news prompted its share price to plummet more than 12 per cent in early trading.