Deputy Rural Fire Officer Jamie Cowan said cold, dry weather dried out dead grass and other fine fuels, making them "highly combustible".
"It may not seem to make sense — it’s a cold, clear day with little wind, so you think it’s a good day to go and light up that pile of branches out in the back paddock [but] what you may not realise is that all these frosts have ‘cured’ [dried out] the grass and you have a much greater risk of your fire escaping than you did at the start of winter."
Winter curing was known to increase the risk of wildfires and, coupled with low humidity, fire lighters needed to be "extra vigilant", he said.
"Things look pretty dead, dull, cold and people go ‘oh well, it must be a pretty good time to light a fire’.
"What they don’t quite realise is that grass around it has been cured by the frost and it’s exceptionally dry.
"They flick a match and think ‘this isn’t going to go much today’ and it goes whoompha and they say ‘why is that?’.
"It’s not until something goes wrong and someone like me comes along and does some checks and says your humidity was at 30% and you wondered why the place exploded."
Winter curing was the cause of a large scrub fire at Hyde in July 2012 when 1500ha of scrub, tussock and forestry was destroyed.
At its peak, the blaze was battled by 40 rural firefighters, staff from Blakely Pacific’s forest plantation and six helicopters with monsoon buckets.
Otago Rural Fire operations manager Graeme Still said the day that fire began humidity — the "invisible factor" — was at just 16%.
"When the humidity is under 50%, you have a far greater chance of the sparks coming off your fire causing ‘spot fires’ out from your main fire.
"When it’s under 30%, conditions become quite explosive and you should think twice before lighting up.
"It [the Hyde burn-off] got away — it was in winter and the frost had cured all the grass ... and away she went.
"That frost cure is just as bad as 30 degree heat."
Both men said farmers needed to keep an eye on the humidity content before they lit a fire and should consider delaying burn-offs if humidity was at or below 30%.
Mr Cowan said the easiest way to check humidity was on weather apps or sites, for example, MetService.
