NSW firefighters have issued a stern warning about heater safety after several people were affected or died from carbon monoxide poisoning in the past month.

On Thursday night, a Surry Hills woman was found conscious but unable to communicate after using an outdoor heater inside her apartment.

The 84-year old is lucky to be alive, after a neighbour happened to check on the woman who was suffering from carbon monoxide poisoning.

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In a string of cases of carbon monoxide poisoning across Sydney during June, a family of four from Penrith suffered carbon monoxide poisoning when they fell asleep around an outdoor charcoal heater which they had dragged into their living room, Fairfax Media reported.

While, a couple died at a weekend camping trip to Kurrajong with police believing the pair suffocated in their sleep as a woodchip fire burnt through the night.

NSW Fire and Rescue Superintendent Paul Johnstone issued an urgent warning about the lethal danger posed by using outdoor heaters inside, after the Surry Hills woman’s brush with death.

“She was using one of those outdoor-type heaters internally for heating purposes. These give off carbon monoxide. Even with the door being opened, the levels (of carbon monoxide) inside were still very elevated and dangerous to a person’s health, and possibly life threatening,” Mr Johnstone told Fairfax Media.

He said people should avoid using outside cooking appliances and heaters indoors, especially in enclosed spaces where there was no ventilation.

For more information about carbon monoxide poisoning visit environment.gov.au.