NSW children who suffer from severe epilepsy will, for the first time in the world, trial a new cannabis-derived drug and have access to a secure supply of medicinal cannabis from early 2016.

Premier Mike Baird announced the NSW government's $3.5 million commitment and partnership with British-based GW Pharmaceuticals on Tuesday, describing the moment as a historic day for the state and for the hundreds of families across NSW who live with the debilitating effects of drug-resistant epilepsy.

"It's a day that I know has given hope to many of these families ... if we can bring relief to these poor children, who wouldn't want to do that?" he said at Randwick Children's Hospital.

Compassionate access to medicinal cannabis Epidiolex, which has shown positive results in epilepsy trials in the United States, will be available from early 2016. A second cannabis-derived drug, CBDV, will for the first time be trialled on epileptic children.