Windows in apartments above ground floor level will have to be fitted with locks to stop them opening too wide, under changes to reduce the high number of children hurt in serious falls.
The rules, announced yesterday by NSW Fair Trading Minister Anthony Roberts, were welcomed by child safety advocates.
In the 2011-12 financial year, 39 children in NSW aged nine or under were hospitalised after falling from windows, many aged under four.
Windows above the ground floor in residential strata schemes will have to be fitted with a lock or safety device preventing the window from opening more than 12.5 centimetres.
Mr Roberts said legislation would be changed and strata corporations and landlords would have five years to comply.
"Adult vigilance is critical when it comes to preventing a child fall, a window safety device is the last line of defence and it could save a child's life," Mr Roberts said.
"The number of high-rise residential apartment buildings in NSW has grown significantly in recent decades with more and more families now living in apartments.
"By 2030 it is estimated that more than 50 per cent of the state's population will live in a strata scheme."
Healthy Cities Illawarra children's health officer Katherine van Weerdenburg said the new rules came after years of lobbying from child safety advocates, and would save lives.
"It's great that they're retrospective because a lot of new legislation [about] child safety is not retrospective - which means older buildings wouldn't have to comply," she said.
She said there might be resistance from interest groups but safety had to be the priority.
"They're practical measures, the evidence is there, they save children's lives," she said.
The rule adds to changes to the Building Code of Australia due to take effect from May 1, which make window safety devices compulsory in new residences.
A paper on children and window safety is at www.fairtrading.nsw.gov.au for comment for six weeks.