The so-called "Urewera Four" were revolutionaries training to establish Tuhoe as its own governing state, the High Court in Auckland has heard.

Tame Iti, Te Rangikaiwhiria Kemara, Emily Bailey and Urs Signer all face charges of participating in an organised criminal group and of unlawful possession of firearms.

Crown prosecutor Ross Burns told the jury during his opening address on Tuesday that the four accused were the organisers of military-style training camps and intended to carry out serious violent offences.

Mr Burns said the camps were held in and around Ruatoki, near Whakatane, each month over a year between 2006 and 2007.

The camps were attended by about 30 people.

Footage taken from hidden cameras was shown to the court of the accused and other people dressed in military-style clothing, heavy boots, scarves and balaclavas and holding guns during the camps.

Mr Burns said the purpose of the camps was to establish Tuhoe as a separate governing state.

The Crown also revealed in court transcripts of text messages, emails and internet chatroom conversations.

In one message, Iti told a person that: "We are a revolutionary wing."

However, Iti's defence lawyer Russell Fairbrother has questioned the accuracy of the Crown's "dramatically presented" evidence and the sources, in his opening address.

Mr Fairbrother said it can't be certain that Iti was using the computer at the time he was alleged to have sent messages on it.

Lawyers for Bailey and Signer have reminded the jury to keep an open mind with Signer's lawyer Christopher Stevenson saying his client was out of the country for a some of the time during which the alleged training camps were held.

The Crown's first witness, the police officer who oversaw the installation of, and retrieval of footage from, surveillance cameras around Ruatoki, began giving evidence on Tuesday afternoon.