"Since 2005, 11 people have died in these types of fires, and we can find no evidence that any of them had a smoke alarm installed. In some cases the fires were quite small but quickly used up the oxygen and generated toxic gases, leading to death," he said.
Smoke alarms could alert people living in the house as well as those inside the sleepout.
"Having a smoke alarm in a sleepout significantly improves someone's chances of getting out alive if there is a fire," Mr Wilding added.
Using a garage as a bedroom was a common practice for many extended families, for teenagers and as a low-cost rental option. However, garage fires grew quickly and were often fed by stored items such as boxes, furnishings and flammable materials such as petrol and solvents.
Mr Wilding said a long-life smoke alarm should be installed in every room used for sleeping, including sleepouts, garages and caravans, even when they were only used occasionally.
Each year in New Zealand there were more than 300 fires in garages, sleepouts and caravans. In 97 per cent of cases no smoke alarms were installed.
In January a 15-year-old boy was lucky to survive a blaze in his sleepout near Ngawha, after he woke to find his bed on fire. The next morning his parents bought smoke alarms for every part of the house.