The New Zealand stock exchange was knocked offline two days in a row due to a cyber-attack.

NZX said it had first been hit by a distributed denial of service (DDoS) attack from abroad, on Tuesday.

The exchange said the attack had "impacted NZX network connectivity" and it had decided to halt trading in cash markets just before 16:00 local time.

Trading halted briefly for a second time, on Wednesday, but was back up and running before the end of the day.

A DDoS attack is a relatively simple type of cyber-attack, in which a large array of computers all try to connect to an online service at once, overwhelming its capacity.

They often use devices compromised by malware the owners do not know are part of the attack.

Genuine traders may have had problems carrying out their business.

But it does not mean any financial or personal information was accessed.