A US man who reportedly wanted to avenge those who died in the Christchurch shootings has been arrested.

Fabjan Alameti, 21, appeared in court on Thursday (US time), NBC News reports, charged with possession of a firearm by an unlawful user of a controlled substance and making false statements involving international and domestic terrorism.

The New York man had earlier allegedly made pro-Islamic State comments on social media, authorities said, and talked about attacking a Jewish temple, "gay club" or military facility.

Alameti was on a bus to Montana when the March 15 massacre in Christchurch took place, authorities said, and asked an informant to send him a link to the video of the attack because it would "fuel" him.

"I will attack random people to avenge the blood," Alameti told the informant according to the FBI, adding that he wouldn't carry out an attack in the US because it had granted him citizenship.

He was later kicked out of a hotel after smoking marijuana, and arrested when he showed up at a firing range a few days later and charged with possessing a weapon while on drugs.

Fifty people died in the Christchurch attacks, by far New Zealand's worst-ever terror attack or mass shooting. The suspect in undergoing a mental health assessment, and will next appear in court in June.

Newshub.