Getting out of onerous clauses in phone contracts, car rental agreements and gym memberships should become easier from Tuesday.

A change to the Fair Trading Act means unfair "fine print" in contracts can be declared illegal.

The law change gives the Commerce Commission the ability to go to court to challenge clauses in so-called "standard-form" contracts if they are unreasonable and create a significant imbalance between companies' and consumers' rights.

One-sided terms and conditions could be declared null and void, even if consumers have knowingly signed on the dotted line.

Wellington lawyer Michael Wigley said the commission had signalled it would move quickly to exercise its new powers and speculated it might make early examples of a telecommunications firm, a gym and a car rental company.

"There will be plenty of business-to-consumer contracts that will be in breach. My take is [the commission] is not going to mess around and we will have action within the next few months," he said. The competition watchdog could go to court after receiving a complaint from a member of the public or on its own initiative.

Contracts that forced consumers, without good reason, to pay early termination fees were likely to be targeted, Wigley said.

The law change will also make it harder for companies to vary the terms of a contract without giving consumers the option to cancel.

The commission issued guidelines in February explaining how it might apply the law.

It said other industries and services that could be effected included car parking, childcare centres, online apps, pay-television agreements, self-storage facilities, the finance sector and residential construction.

Insurance contracts have been specifically excluded from the law change.

Any contracts that have been individually negotiated between a company and a customer are also not covered.

Consumer NZ said it had campaigned for the law change for several years.