As many as 30 people have attended Royal Darwin Hospital after taking contaminated drugs at a party overnight, St Johns Ambulance says.

Police say they began receiving reports about 7pm of people collapsing, behaving irrationally, vomiting and in semi-consciousness.

"Investigations revealed that all of those affected had attended a party at an apartment in the city," Duty Superintendent Bob Rennie.

"At that party there were drugs being handed out and a number of people have taken those drugs.

"Just after 7pm we started having people dropping all over the place and behaving very erratically.

"We had one unconscious.

"We were at a loss as to what we had initially.

"We believe there may have been a person in the inner city selling contaminated drugs."

St John Ambulance operations manager Craig Garraway said seven people were taken by ambulance to the hospital.

Mr Garraway said he had heard between 30 and 35 people affected by the drugs may have presented to the hospital themselves.

He said he had never seen anything like it in his 20 years working in Darwin.

Six people remain in hospital, with two of them sedated and in intensive care.

Duty Superintendent Rennie said police were not yet sure what kind of drugs were involved.

"We do have possession of some of these capsules that some of these victims had and they will have to be checked out by our forensic section and analysed."

Following the emergency, police entered CBD nightclubs and issued warnings on loudspeakers to others who may have attended the party.

The Major Crime Squad is investigating the matter and warning anyone who received drugs last night not to take them.

Duty Superintendent Rennie said if people choose to take party drugs, they are playing Russian roulette with their lives.

"They just don't know what they are taking."