The finding, released this afternoon, gives greater substance to claims by the internet tycoon that his phone is being tapped and his calls monitored.
But Dotcom did not get the court ruling he sought, having asked extradition Judge Nevin Dawson to order the United States to rule out any form of surveillance on him.
Dotcom is facing extradition to the United States - along with three others - after the FBI shut down his Megaupload website. The four men face charges of criminal copyright conspiracy.
The current ruling came after he had a phone call with lawyer William Akel interrupted and then heard his voice played back for 15 seconds.
Other evidence put to an Auckland District Court hearing included markedly improved mobile phone reception at his Coatesville mansion despite the network provider making no changes. The concerns dated back to September 2011.
Judge Dawson said: "On the evidence before this court it would appear likely that some form of surveillance and/or interference with telephone communications has happened.''
It did not mean it was certain surveillance had happened, he said.
Judge Dawson also refused to make the order against the US, saying Dotcom would need to produce evidence showing the country was involved.
The application to the court was accompanied by expert evidence showing the type of technology needed to intercept the calls was in use by the FBI. A device called Stingray pretended to be a cell tower - boosting performance in the affected area - while tricking cellphones to run their communications through it.
Police headquarters last week refused to confirm whether it also used the technology.
A spokesman said: "Based on the evidence presented in the court to date it is unlikely that Police would consider an investigation into these allegations.''
Dotcom has previously been illegally spied on by the Government Communications Security Bureau, leading to an apology from Prime Minister John Key.
His extradition hearing is in April.
