The head of spy agency NZSIS says she knows the spies must do better after March 15 when her agency's terror watchlist was focussed solely on Islamist extremism.

But she still couldn't assure the public that the spies were looking in the right places and picking up obvious far-right threats.

A bomb threat targeting Al Noor and Linwood mosques this March 15 was only foiled after the public called it in - having spotted it on website 4chan.

Rebecca Kitteridge, Director-General of the New Zealand Security Intelligence Service (NZSIS) appeared before the Intelligence and Security Committee on Wednesday, for an annual grilling by Parliament's most senior MPs. 

Her appearance at Parliament followed a major report this week which called the spies out to do better at detecting security threats. 

"We don't monitor the entire internet," Kitteridge told the committee. "We are not conducting mass surveillance of New Zealanders." 

But no one's asking them to monitor everyone - just those who pose a threat.

When asked by Newshub why the NZSIS isn't monitoring sites like 4chan, Kitteridge said the agency's activities "won't be visible to the New Zealand public and that's who it has to be". 

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She added: "We won't be telling you where we are active and where we are not."

The Royal Commission into the mosque attacks showed that before March 15 2019, zero right-wing extremists were on the terror watchlist while 100 percent of investigations were into Islamist extremism.

Green MP Golriz Ghahraman expressed disappointment in Kitteridge's responses. 

"I actually heard her defend her agency and deny the finding that essentially - let's call it what is - racism had been happening and operating under her watch," Ghahraman said. 

Kitteridge said it has been a time of "deep reflection" for the NZSIS since March 15 2019. 

"We must do better and we will," she told the committee. 

Kitteridge said her agency's counter-terror effort is split 50/50, but she couldn't say if that extends to the actual terrorist watchlist.

"Our effort is about 50/50," she said. 

The NZSIS is testing the waters with politicians to be allowed to do more data-mining to prevent attacks like March 15 - scooping up your private data. 

Until it can prove it is focusing its investigations in the right places - picking up obvious threats - that could be a hard sell with the public.