Paul Edmonds, a solicitor at Canberra Criminal Lawyers, said no legislation had passed ensuring there was a dedicated area where participants in the trial wouldn't be held criminally liable for drug possession.

"All we have is an unofficial agreement that festivalgoers won't be arrested for the possession of drugs," he said.

"As far as I'm aware, no legislation has passed to provide any clear legal protection to pill users at the event."

The solicitor said the pill-testing trial could also present liability issues if participants were to suffer any adverse affects from drug taking after they had taken part in the trial.

"It depends on what sort of written indemnity form that the pill user signs before they have the pill tested. There would obviously be some kind of waiver," he said.

"However, an issue could arise if the person approaching the pill-testing tent is already under the influence, and would they be able to give informed consent and understand what they are signing up to."

The pill-testing trial comes after a last-minute breakthrough in negotiations on Thursday when the event's promoter Cattleyard Promotions gave approval.

Both the ACT government, ACT police as well as medical experts had already given their backing to the proposal, which will be conducted in a tent operated by Safety Testing Advisory Service at Festival and Events (STA-SAFE).

The trial was expected to go ahead at last year's Spilt Milk festival, but was scrapped at the last minute.

Mr Edmonds said while in favour of a harm-minimisation approach, the pill-testing trial could have unintended legal ramifications if an overdose was to occur.

"My concern is around whether the legal machinery around the trial is appropriate, or whether it's simply merely an ad hoc approach that leaves open the possibility of all sorts of legal consequences," he said.

A media ban on advocates and organisers of the trial has meant the trial will go ahead on Sunday without key pieces of information being public.

The ban was put in place by the festival's promoter and is expected to lift at the end of the event.

An ACT Policing spokesman said a "health precinct" would be set up inside the venue where the pill testing would take place, and that this location wouldn't be targeted by officers.

"ACT Policing will not actively target the health precinct or their users in line with well-established protocols with ACT Health facilities," the spokesman said.

"As a police force, we will continue to target and investigate the sale and supply of illicit drugs."

It is not known at this stage whether plans are in place to use sniffer dogs to search festivalgoers as they enter the venue, or if drug testing will take place outside.