NZX Ltd was halted for most of last week due to the attacks, which authorities have said originated offshore.
Monday’s attack came shortly after NZX said it had agreed with the Financial Markets Authority (FMA) on a back-up plan for the release of market announcements.
A spokesman for NZX confirmed the website was down, but said trading on its platform, which began at 10.00 a.m. local time (2200 GMT), was continuing as usual through the contingency arrangements.
He declined to comment on who was behind the attacks, whether there had been any extortion demand or what measures have been put in place to stop future attacks.
The bourse said in a statement that the exchange was working with its network’s service provider, Spark, government cybersecurity agencies, and a U.S.-based cybersecurity firm Akamai Technologies to implement additional security measures.
“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,” NZX’s Chief Executive, Mark Peterson said.
The market was nearly flat at 0230 GMT.
NZX Ltd faced disruptions for four days last week when it was hit repeatedly by distributed denial of service (DDoS) attacks, a common way to disrupt a server by overwhelming it with a flood of internet traffic.
The attacks forced NZX to halt trading in its cash markets, disrupting operations in its debt market, Fonterra Shareholders Market and derivatives market.
NZX said its core trading platforms were not hit in last week’s attacks but the public facing website crashed, affecting the exchange’s ability to publish market announcements.
“At those times, NZX made the decision to halt the market in order to maintain market integrity,” it said.
News websites of Stuff and Radio New Zealand had also been targetted by cyber attacks, local media reported on Monday, but the attacks were unsuccessful.
New Zealand, a relatively small economy with a population of five million, is not often the target of such attacks but neighbouring Australia ramped up its cyber security this year after a rise in similar incidents.
Finance Minister Grant Robertson said last week the Government Communications Security Bureau and the national agency fighting cyber crime had been called in to help the bourse.
NZX said the National Cyber Security Centre (NCSC) has sent a warning notice to New Zealand companies about the attacks.