A hay fire that burned through the night, sending smoke across Wagga suburbs, is expected to smoulder for a number of days. About 1600 hay bales were in a 50x20-metre shed when it the fire started at a property on Inglewood Road in Forest Hill on Thursday afternoon. NSW Rural Fire Service (RFS) volunteers were called in to tackle the blaze about 2.50pm and were assisted by Fire and Rescue NSW personnel. RFS Riverina operational officer Marty Boyce said there were early concerns about the fire spreading but crews were able to contain it. However, the crews have not been able to extinguish the fire due to the extreme heat and fears the shed would collapse. "You have a significant number of square bales, so a significant amount of heat was generated," Mr Boyce said. "The guys had a thermal camera on it and measured some parts of it up to 1200 degrees. "It could go for days - we'll be patrolling it, monitoring it and, when we can, we'll get some machinery out to put it out." Smoke warnings were issued for the neighbouring areas of Forest Hill and Gumly, which are expected to remain in place until the fire is extinguished. People in the area affected by the smoke have been advised to stay inside and close windows and doors. Mr Boyce said there was likely to be additional smoke when crews were able to break the bales apart safely in order to extinguish the fire. Hay fires from spontaneous combustion are common at this time of year. The fires are started by a mix of chemical and microbial reactions that occur in damp hay. The density of the fuel makes them particularly hot and challenging to put out. Bales usually need to be broken up to be extinguished. There have been 10 hay fires in the vicinity of Wagga in recent weeks and about 30 across the region. Mr Boyce said you never know which bale is going to start a fire. "They generate heat from inside the stack," he said. "You can never tell which bale is going to be the one to ignite ... but it's really about monitoring the temperature inside. "There's an old trick where you put a crowbar into a couple of bales, and if you can't touch it, it's too hot."