A decontamination expert has called for testing for meth residue as part of planned rental housing warrant of fitness tests, saying families are suffering ill effects from former P houses.
Northland police have uncovered 13 dwellings used as P labs since 2011.
Figures released under the Official Information Act show they discovered four clandestine P laboratories in 2013, equal to 2012 and down from five in 2011.
Meth Solutions tested 53 Northland properties for traces of methamphetamine last year.
Of those, 55 per cent were contaminated and the level of contamination in eight suggested meth had been manufactured on the premises, according to director Miles Stratford.
"A lot of the people we are dealing with are tenants who have moved into a property, and then they have got the test done because they have started to feel sick."
Short-term effects of meth exposure included headaches, sore eyes, or increased symptoms of asthma or eczema.
Long-term effects included stomach aches, body pains, dizziness, a metallic taste in the mouth or paranoia.
Children in affected properties were "basically running around wired on low doses of methamphetamine", Mr Stratford said.
The "real kicker" was an increased risk of cancer or deterioration of the nervous system, he said.
Nationwide, police uncovered 62 dwellings used as P labs last year, down from 76 in 2012.
Despite the health risks from toxic chemicals used to manufacture the drug, children were found living at two Northland P lab dwellings last year and 23 nationwide.
Whangarei District Council regulatory services manager Grant Couchman said that, when police referred a contaminated property for cleaning, the council served a meth testing notice on the owner.
If contamination was present, the property had to be professionally decontaminated and a record of the process was included in the property's LIM report, he said.
All the properties referred from police over the past few years were rentals, he said.
Police Clandestine Laboratories Response Team acting national manager Detective Senior Sergeant Graeme Anderson said signs a property was being used to manufacture meth included blacked out windows, strong chemical smells or significant chemical purchases.
One of the most disturbing scenes was the presence of children in P lab properties, Mr Anderson said.