The New Zealand stock exchange has been shut down because of a serious cyber attack from overseas.

Cash market trading was halted after an attack took down the NZX website just before 11.30am local time (9.30am AEST).

After about half an hour, trading resumed.

"NZX confirms that it is working with its network service provider to address a further issue today, impacting NZX system connectivity," the stock exchange said in a statement.

"It appears that this is similar to yesterday's issue. NZX halted trading in its cash markets at approximately 11.24am.

"NZX's network provider continues to investigate the source of the issue."

It came the day after a distributed denial of service (DDoS) attack caused a major power outage before the close of business yesterday.

Trading stopped at 3.57pm yesterday because of the attack.

NZX confirmed the attack came from outside New Zealand, but did not specify where.

A DDoS attack is a method of shutting down a website by flooding it with an overwhelming volume of traffic.

The traffic typically comes from a network of computers infected with malware, which are directed by an attacker to flood any given website.

The NZ50 Index opened slightly down the morning after a positive start to the week.

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Source: 9News