Judge Behrens agreed.
"I find that New Zealand citizens have an expectation that police will not come on to their land to randomly breath test them," he said.
He accepted there was public interest in catching drink-drivers, but said public interest did not override the reasonable expectation of privacy.
Police were today considering appealing the judge's decision.
In the meantime, West Coast area commander Inspector John Canning said people should not read that decision as an escape clause from possible drink-driving convictions.
"I wouldn't advise ignoring the red and blue flashing lights and then driving on a private property as a good course of action," he said.
"If people do that they will be promptly arrested for ignoring the flashing lights and then put through the drink-driving procedures."
The file was today back at Tasman police headquarters but chief prosecutor, Senior Sergeant Tony Bowen, said it was too early to say whether an appeal would be lodged. He would discuss the case with others in the Crown legal team before coming to a conclusion.
Police had contended that Taffs had failed to stop for the police car's flashing lights, but Judge Behrens said there was no evidence he saw the lights before turning into his driveway.
"The inference I draw is that constable (Greg) Sherie was intent on beginning breath testing procedures because the decision to do so had been made before he entered on to Mr Taffs' property.
"He did not consider whether he could go on to the property or not. I found him to be a combative and argumentative witness."
