Several changes to the ACT's tenancy laws came into effect last week, a year after reforms passed the legislative assembly, that will affect the contracts Canberra's 112,000-odd renters have with their landlords.

Among the changes now in force for all new tenancy agreements was a new "opt-in" clause for lease breaking that would see renters pay six weeks' rent if their lease is less than three years long and they break the lease less than one and half years into it.

If the tenant breaks the lease more than halfway into a three-year lease, the amount they would owe landlords was four weeks' rent, while for leases longer than three years, the figure needs to be agreed between the tenant and owner.

Tenants Union ACT's Deb Pippen said while the new clause was not mandatory, she was concerned many landlords would be forcing the clauses on renters.

Ms Pippen said that under the previous laws, renters were potentially liable for up to six months rent when they broke a lease, and the changes were to help create certainty for renters and landlords.

"The problem is that in a tight rental market like we have now, that if you're moving out and they advertise properly or you have someone to replace you on the lease, tenants can limit it to a couple of days or at most a months rent," she said.

"But under this clause, even if there's someone to replace the tenant with immediately, you're still looking at six weeks rent if they break the lease, so there isn't actually a benefit for the tenant if you agree to this."

She said when the old lease breaking clauses had been tested in the ACT Civil and Administrative Tribunal, she was not aware of any cases that the Tribunal had awarded landlords more than four weeks' rent.

Other changes now in force include that all properties must have smoke alarms installed under any new contracts, and owners of properties under existing leases have until August next year to install smoke alarms if they are not already installed.

Ms Pippen also welcomed new "opt-in posting clauses", that extend notification periods for renters and owners from four weeks to eight weeks, giving renters and owners more time to prepare for changes.

She said there were also other changes affecting bonds over properties that should ensure bonds, and any related disputes, are more quickly sorted out, with a 10-day notification period, which applies to existing and future tenancy agreements.

The changes also create new provisions under the Act to allow for tenants experiencing domestic violence to end a tenancy agreement, or have the perpetrator removed from the lease, by applying to the Tribunal.

Ms Pippen said those changes would help to protect victims of domestic violence, but that the bar was set "quite high" for victims to provide enough evidence of such violence to the Tribunal in such cases.