Police said synthetic cannabis, which was outlawed in New Zealand in 2014, was often laced with unknown substances and could cause fatal seizures.
"We have grave concerns as users don't know what poisonous chemicals they are potentially putting into their bodies when they're smoking this drug," detective inspector Gary Lendrum said in a statement.
Police did not supply further details of the seven deaths in New Zealand's largest city but said all the cases had been referred to the coroner.
Chief coroner Deborah Marshall said ambulance services had also reported a significant number of non-fatal cases where people had been hospitalised after using the drug.
New Zealand banned synthetic drugs three years ago, meaning supply or manufacture can result in a two-year jail term or fine of up to NZ$500,000 ($371,000).
Man-made cannabinoids are either sprayed on shredded dried plant material to be smoked, or sold as a liquid which can be "vaped" in e-cigarettes and other devices.
Synthetic cannabis can include psycho-active ingredients many times stronger than those naturally found in marijuana.
It can also contain other highly toxic substances, with its chemical make-up varying from batch to batch, with unpredictable side effects.
The UN's drugs control body in 2015 warned that new synthetic drugs, which emerged in the United States about a decade ago, posed a serious health problem.
Cities like New York have launched active campaigns against synthetic cannabis, which has killed several people and sent thousands of others to emergency rooms.
