A woman has been charged with murder over the 2015 death of a toddler who was in her care.

The 20-month-old boy died at hospital on March 23 that year, and police began investigating.

Emergency services had been called to a home in Neilrex, south of Coonabarabran, following reports the boy was coughing and not breathing at 5.15am on the day he died.

He was taken to Coolah hospital, where he was pronounced dead.

The 40-year-old woman was arrested on Monday afternoon in Geurie, near Dubbo, and taken to Dubbo Police Station where she was charged with murder.

The woman was refused bail. She will appear in Dubbo Local Court on Tuesday.

With AAP