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Labour MP Ross Robertson says he twice told police about a "tinnie house" operating next door to his Otara electorate office.

Mr Robertson said he had notified the authorities as soon as he found out about the drug dealing.

Police raided the property, and he notified them again when the cannabis dealing continued. He said the tinnie house was set up this year.

"It hasn't been there that long. It's not as if it's been there for a year or six months."

Mr Robertson was responding to a Herald on Sunday story which reported that 40 people entered the house in two and a half hours on one day last week.

The paper said the house had been raided once despite a string of complaints to police. One man was arrested on a drugs-related charge after the raid.

National Party law and order spokesman Simon Power said the failure to carry out another raid despite the complaints was not good enough.

"We must stamp out these drug dealers. Getting rid of them must be a higher priority for the police than it seems to have been in this case," Mr Power said.

"I would like the police minister to explain what happened here."

Mr Robertson said he could do no more than alert police. "Once I've given them the information, I can't do anything else. It's in their court. I'm not a police officer."

He said he worked with police on issues such as gangs in the electorate, but a drug bust was an operational matter for police alone, and it was up to them to decide when to act on information they were given.

"They have got operations going on all the time. I'm sure there are other houses they're monitoring."

Mr Robertson said he could not remember exactly when he last notified police, because he had a heavy workload.

He used the office on Friday afternoons, because he shared his time between electorate offices in Papatoetoe, Otara and Meadowlands near Howick.

A spokesman for Police Minister Annette King said she would not comment on operational matters.