Warnings are being issued after suspicious behaviour by people claiming to be tradespeople in and around Christchurch.
Police and the Master Builders Federation are urging people to ask tradespeople for references and qualifications before starting building work.
Police believe opportunists are trying to take advantage of vulnerable residents, such as an elderly woman who was approached by a man offering to take down her chimney.
They say the man had no logo on his clothing or work van, acted suspiciously and asked for much more money than the job would have cost.
A number of other people have reported similar incidents, police say, and they urge residents to ask for identification and to make sure they inquire about trade qualifications.
Master Builders Federation chief executive Warwick Quinn has been fielding calls from concerned home-owners and also urges residents to be extra careful and check out people wanting to do work for them.
The federation says amendments to the consenting process expected to be announced this week, aimed at speeding up earthquake recovery, may help foil opportunists by specifying which tradespeople can be used.
Christchurch City Council also says changes to fast-track consents for rebuilding will stop cowboys exploiting the disaster.
Taking advantage of welfare centre
Opportunists have been evicted from an earthquake welfare centre in Christchurch after it was established they were taking advantage and had not been affected by the quake.
Canterbury police district commander Superintendent Dave Cliff says about 30 people were creating a nuisance at the Addington Welfare Centre on Saturday night.
Their homes had been assessed as safe but they refused to move out of the centre until they were evicted under the Trespass Act.
Supt Cliff says police will continue to maintain a presence at the welfare centres for some time.