"If cannabis becomes legal, our members need to know exactly where they stand on recreational use themselves."
Speaking to reporters at the conference, Nash said he did not want stoned police officers turning up to work.
"I'd be horrified if officers started turning up stoned, as I would be if they turned up drunk.
"Alcohol is legal. You wouldn't expect an officer to turn up drunk. If marijuana did become legal, you'd certainly not expect an officer to turn up stoned either."
Cahill said cannabis stayed in the system a lot longer than other drugs, including alcohol, and an officer or a driver could use cannabis and not be impaired.
"Defining impairment is going to take serious scientific and legal input, and the sooner we start, the better.
"We cannot have [police] turning up to work believing they are clean only to test positive for THC in a critical incident that may have occurred days after use of cannabis."
Cahill pointed out several issues for debate to the conference, including how to define personal use and an age limit - especially in light of research about harmful brain development in young people - and how to legally access cannabis.
He also said that everyday firearms shootings were now the most serious safety issue for police.
"Only weeks ago, we were all jolted when an offender allegedly aimed a cut-down rifle at two officers and pulled the trigger twice. Through sheer luck, the gun's safety catch was on.
"The potential dangers to us all are stark."
Answering questions from the conference floor, Nash did not offer an opinion on the general arming of police, saying it was an operational decision for the Police Commissioner.
Cahill also recommended that association members accept the Government's latest pay-rise offer of 3 per cent per year for the next three years.
"We don't believe there is a better offer coming."
Cahill thanked the Government for extra police recruits, though was concerned about the loss of experienced officers.
Last night it emerged that four police recruits from Wing 318 had yet to graduate pending investigations for misconduct, and Cahill gave assurances that new recruits had to pass "very high" standards of behaviour.
Nash said the Government would not drop standards for police recruits, even if it meant missing the goal of adding 1800 extra officers over three years.
"If on the off chance we just don't get the good-quality men and women, it'll be three and a half years."
But he rejected the idea that the Government was looking at extending the time period from three to five years.