The New Zealand stock exchange has crashed for the third day in a row under the weight of a concerted Distributed Denial of Service attack.

NZX was forced to halt trading in its cash markets at around 11:10am and then made the decision to not re-open the NZX Main Board, the NZX Debt Market and Fonterra Shareholders’ Market for the rest of the trading day and closed the NZX Derivatives Market.

In a letter to market participants, the exchange operator said: "This decision not to re-open has been made while we focus on addressing the situation. We continue to address the threat and work with cybersecurity experts, and we are doing everything we can to resume normal trading tomorrow."

The Exchange was out of action for almost four hours on Wednesday as hackers flooded its servers with traffic at 11.24am. A similar attack on Tuesday shut down trading for the last hour of the day.

The hacker groups behind the attacks are believed to be demanding bitcoin payouts to cease the DDos overload.