WELLINGTON: Cyberattacks continued against the New Zealand stock exchange on Monday but failed to force a closure of the market after it shut down four times last week, operator NZX said.

While the foreign-sourced attacks forced NZX's website offline, the company said the bourse itself was trading as normal.

Chief executive Mark Peterson said US cyber-defence experts Akamai Technologies had been brought in to help repel the attacks, alongside network provider Spark and New Zealand's GCSB spy agency.

"NZX has been advised by independent cyber specialists that the attacks last week are among the largest, most well-resourced and sophisticated they have ever seen in New Zealand," he said.

NZX said it had agreed on protocols with regulators at the Financial Markets Authority to give investors access to market announcements and allow the market to remain open, even if the NZX website was down.

It did not specify how investors would access such market-moving data.

NZX has been tight-lipped about who is behind the attacks.

Experts have told local media that possible perpetrators include state-sponsored actors, online activists with an anti-capitalist agenda such as Anonymous, or a criminal enterprise seeking a ransom.

The cyberattacks experienced by NZX are called distributed denial of service (DDoS) attacks, which bombard systems with data requests or traffic.

Akamai said in a blog post earlier this month that cybercrime groups calling themselves Fancy Bear and Armada Collective had been warning companies in the finance, travel and e-commerce sectors they would face DDoS attacks unless they paid a ransom.