A police officer said three people - two females and one male - had been transported to hospital after ingesting or inhaling some sort of substance.
"We think they've ingested or smoked something and obviously OD'ed on something and they've gone to hospital to hopefully come round.
"My guess is it would be some sort of synthetic cannabinoid."
The patients had been found directly outside SkyCity's main entrance. They would now undergo toxicology tests at hospital to confirm what substance they had taken.
Kai Laidler, a staff member at United Coffee Nation on Federal St, said a woman buying her morning coffee "just ditched it" about 9am after seeing someone being dragged outside the shop.
"She went outside and people were helping someone and then four ambulances came and cops came. Another customer came in 20 minutes later and said they'd had a seizure."
A man sitting near SkyCity asking for spare change said he understood the trio had been smoking synthetic drugs.
"They just passed out. It wasn't good. They were incoherent, put it that way."
The man said security guards erected screens around the three people while they waited for ambulances to arrive.
The man told the Herald he had lost his own brother to synthetics in 2018.