Trials of medical cannabis get underway in NSW this week but it could take years before it's approved for mainstream use.

The $9 million trials will be conducted on children with severe epilepsy, terminally ill adults and those undergoing chemotherapy.

Two of the trials will be undertaken at The Childrens Hospital at Westmead and Sydney Childrens Hospital.

But while hundreds of patients will be accepted into the program, those who miss out could face a lengthy wait to experience any therapeutic benefits.

"Medical cannabis' time has come," Premier Mike Baird told reporters.

Mr Baird stressed the government was proceeding with caution.

"We need to get the research in place, we need to get the scientific evidence in place, we need to get the medical evidence in place and once we have that we can work out how we can make a difference to thousands of people," he said.

Health Minister Jillian Skinner said it could take years for the trials to be approved for mainstream use.

"Clinical trials have to go through very rigorous ethics standards, controls have to be approved by the therapeutic goods association, which is a Commonwealth body, so it could take months if not years," Ms Skinner said.

And new guidelines have been given to police who now have the discretion to look the other way with terminally-ill cannabis users and their carers.

NSW Premier Mike Baird. (AAP Image/Paul Miller) ((AAP Image/Paul Miller))

An expert panel led by NSW Chief Medical Officer Dr Kerry Chant has been established.

The head of the epilepsy program at The Childrens Hospital at Westmead, Dr Deepak Gill, and child neurology epilepsy specialist Dr John Lawson from Sydney Childrens Hospital, Randwick, and their teams will also be involved in the trial.

But deputy opposition leader Adam Seale said the trials are moving too slowly and the new police guidelines are creating grey areas in the law.

"The terminally ill have little to look forward to from these clinical trials, which could take years that many people simply do not have to wait," Mr Seale said.

Before the trials can get underway the NSW government has to overcome logistical hurdles in importing medical cannabis products from Europe or the US.

Approval for the importation of the products may take several weeks, but if permission is denied the NSW government has indicated it could grow its own cannabis crop.

The government could also outsource the task to the private sector.

Supply and distribution protocols will be established for each trial and Mr Baird stressed that the drug remains illegal.

"The government will continue to reinforce the message that recreational use of cannabis is illegal and will not be tolerated," Mr Baird said.

Australian Medical Association NSW president Saxon Smith welcomed the trials.

"This will allow sick people access to medicine that is being tested and examined for efficacy - potentially improving their lives in the short term and providing evidence to help the wider population in the longer term," Dr Smith said.