Jordan William Thompson was 21 months old when his mother, Bernice Swales, left their Singleton unit during the afternoon of Thursday, March 19, 2005 to run some errands.

She left her young son in the care of her then partner.

Police said when Ms Swales returned home, Jordan was "unresponsive".

Ms Swales rushed Jordan across the road to Singleton Base Hospital, but the toddler could not be revived.

"Police were told Jordan had been found face down in the bathtub; however, a post-mortem later found high levels of a prescription medication used to treat depression in his system," NSW Police said in a statement on Monday.

Detectives from the State Crime Command's Homicide Squad established Strike Force Kyeama to investigate the circumstances surrounding Jordan's death.

A Coronial Inquest commenced in 2008 and resumed in 2015, with Deputy State Coroner Elaine Truscott finding in 2018 that the toddler had died because they had been deliberately administered anti-depressant medication.

Following further extensive investigations, and a NSW Government $100,000 reward for information, Strike Force Kyeama detectives arrested a 49-year-old man at a home at Doonside about 5pm on Monday, September 27.

The man was taken to Blacktown Police Station where he was charged with manslaughter.

The 49-year-old man was refused bail on Monday and was due to face court in Sydney's west on Tuesday, September 28.