WELLINGTON • New Zealand's stock exchange was halted for more than three hours yesterday as it came under cyber attack for a second day.

Exchange operator NZX halted its cash markets at 11.24am in Wellington and trading did not resume until 3pm.

It said in a brief statement that yesterday's issue was "similar" to Tuesday's cyber attack, which disrupted the final hour of trading.

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 Dr 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."

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 was able to mitigate the issue and the market began as normal at 10am yesterday before prices and index updates were again halted.

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

There is no indication yet as to who might have mounted the attack.

BLOOMBERG