New Zealand’s stock market has been interrupted by an apparent overseas cyber attack for the second day running.

The Wellington-based NZX exchange went offline at 11.24am on Wednesday and although some connectivity was restored for investors, some trading was halted.

The NZX said it had experienced “network connectivity issues” and that the NZX main board, NZX debt market and Fonterra shareholders market were placed on halt.

However it then announced that those areas would resume trading with the rest of the market at 3pm on Wednesday.

The interruption followed a shutdown and trading halt on Tuesday afternoon due to an overseas-based distributed denial of service (DDoS) attack.

The incident follows a number of alleged cyber attacks by foreign actors, such as the targeting of a range of government and private-sector organisations in Australia.

In a statement earlier on Wednesday, the NZX blamed Tuesday’s attack on overseas hackers, saying that it had “experienced a volumetric DDoS attack from offshore via its network service provider, which impacted NZX network connectivity”.

It said the attack had affected NZX websites and the markets announcement platform, causing it to call a trading halt at 3.57pm.

It said the attack had been “mitigated” and that normal market operations would resume on Wednesday, but this subsequent attack has raised questions about security.

A DDoS attack aims to overload traffic to internet sites by infecting large numbers of computers with malware that bombards the targeted site with requests for access.

Prof Dave Parry, of the computer science department at Auckland University of Technology, said it was a “very serious attack” on New Zealand’s critical infrastructure.

He warned that it showed a “rare” level of sophistication and determination, and also flagged security issues possibly caused by so many people working from home. “Unfortunately the skills and software to do this are widely available and the disruption of Covid and people working from home all over the world potentially with lower security on their computers means that these attacks are easier than usual.”