NEW Zealand's stock exchange was halted for more than three hours on Wednesday as it came under cyber attack for a second day.

Exchange operator NZX halted its cash markets at approximately 11.24 am in Wellington and trading didn't resume until 3pm. The day's issue was "similar" to the previous day's cyber attack that disrupted the final hour of trading, NZX said in a brief statement. The NZX website and feed of company announcements also went down, but have since been restored.

"This is a very serious attack on critical infrastructure in New Zealand," said Dave Parry, a professor of computer science at Auckland University of Technology. "The fact that this has happened on a second day indicates a level of sophistication and determination which is relatively rare."

Cyber attacks aren't common in New Zealand but in neighbouring Australia there has been an increase in incidents. In June, Australian Prime Minister Scott Morrison warned that the frequency of attacks on government, health and education services and various industries has been climbing over many months, and said organisations needed to improve their cyber resilience.

On Tuesday, NZX said trading was disrupted by a "volumetric distributed denial of service attack from offshore" - an attempt to disrupt service by saturating a network with significant volumes of Internet traffic. It had been able to mitigate the issue and the market began as normal at 10 am on Wednesday before prices and index updates were again halted.

The S&P/NZX-50 benchmark index was near a record high when Wednesday's incident occurred. It was at 11,995 at 4.23pm, little changed from Tuesday's close.

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"NZX is an important part of New Zealand's financial system," said Shane Solly, a portfolio manager at Harbour Asset Management in Auckland. "The NZX has worked hard to resolve the outage, but it would be concerning if the outages continued." There is no indication yet as to who might have mounted the attack.

"NZX's network provider continues to investigate the source of the issue. We will provide further information once available," NZX said. BLOOMBERG