A lawyer for three activists who attacked a dome at the Waihopai spy base has questioned the legality of the facility's establishment.
The men are appealing against a High Court ruling that they were liable to pay for the damage they caused in 2008.
The Crown is seeking more than $1.2 million from Adrian Leason, Father Peter Murnane and Sam Land.
Their lawyers told the Court of Appeal on Wednesday that a full civil trial should be held to determine whether they are liable for damages, given that in 2010 a jury acquitted them on charges of criminal damage.
One of their lawyers said the Crown acquired the land at Waihopai for defence purposes, but the Government Communications Security Bureau (GCSB), which runs the spy base, isn't part of the Defence Force.
He said Waihopai is supposed to contribute to New Zealand's national security, but it is a conduit to supply intelligence to the United States.
The court was told that five years after the Waihopai attack, it has still not been established that the men committed an unlawful act.
Lawyer Michael Knowles said there is no reason why the GCSB should be exempt from the requirement to prove its case and counter his clients' defence of self-defence or defence of another.
The Crown will make its submissions on Thursday.