Ministry of Health guidelines do not identify any safe level of Meth contamination, and guidelines around the world vary. In New Zealand, decontamination is recommended, if 0.5 micrograms (0.0005g) of Meth are detected in one 10cm by 10cm area. If detected, your local authority has powers under the Health Act 1956 to order cleansing of the property and could place a permanent requisition on your property file.
The chemicals used to cook Meth, and the residue from its use, can be highly toxic and can linger for a long time after being absorbed into housing materials. Health risks include burns and respiratory & neurological damage. Decontamination can require complete redecoration to the affected area, including replacement of carpets, curtains and wall linings.