The department has been tipped off that roving scammers have been offering cheap rates on various home improvement services such as bitumen driveway sealing, roof repairs and painting and other large household projects.

"These types of traders will often imply they have leftover materials from a nearby job and offer a cheap price for work carried out that day," Fair Trading Commissioner Rose Webb said.

The groups are believed to be largely English or Irish, the commissioner said, and any sightings of such approaches should be reported to NSW Fair Trading.

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Anyone who feels threatened by salespeople who intimidate or make threats about payment should call police immediately, Fair Trading said.

Under Australian consumer law, anyone who doorknocks or cold calls NSW residents and offers work valued at more than $100 must inform consumers of their rights to terminate agreements as well as a 10-business day cooling-off period.

Anyone carrying out work valued at more than $5000 on a residential property must be licensed, and consumers should ask to see that licence. They can be checked online through fairtrading.nsw.gov.au.