The results showed a three-fold increase in effort after over-drinking.
"Here for the first time we found effort-full swallowing after drinking excess water which meant they were having to overcome some sort of resistance," Associate Professor Farrell said.
"This was compatible with our notion that the swallowing reflex becomes inhibited once enough water has been drunk."
Associate Professor Farrell, who works in the Monash University Department of Medical Imaging and Radiation Sciences, used functional magnetic resonance imaging (fMRI) to measure activity in various parts of the brain, focusing on the brief period just before swallowing.
The fMRI showed the right prefrontal areas of the brain were much more active when participants were trying to swallow with much effort, suggesting the frontal cortex steps in to override the swallowing inhibition so drinking could occur according to the researchers' instructions.
"There have been cases when athletes in marathons were told to load up with water and died, in certain circumstances, because they slavishly followed these recommendations and drank far in excess of need," he said.
Drinking too much water in the body puts it in danger of water intoxication or hyponatremia, when vital levels of sodium in the blood become abnormally low potentially causing symptoms ranging from lethargy and nausea to convulsions and coma.
Associate Professor Farrell said elderly people, however, often didn't drink enough and should watch their intake of fluids.