The exchange went offline at 11.24am and was still offline as midday approached.
"NZX confirms that it is working with its network service provider [Spark] to address a further issue today, impacting NZX system connectivity. It appears that this is similar to yesterday's issue," a spokesman said.
Yesterday, trading was halted for an hour in the afternoon.
In a joint statement with Spark, NZX said yesterday it "experienced a volumetric DDoS (distributed denial of service) attack from offshore, which impacted NZX system connectivity".
"As such, NZX decided to halt trading in its cash markets at approximately [3.57pm]. A DDoS attack aims to disrupt service by saturating a network with significant volumes of internet traffic."
The statement added that the attack "was able to be mitigated and connectivity has now been restored for NZX".
