Two other boys, aged 9 and 10, were hospitalised last week after they were found vomiting and unresponsive in a park at night.
Mr Middleton said the pair, who were both in the care of Child, Youth and Family, were forced to smoke synthetic cannabis by older youths.
"We see extreme forms of violence. People who use it become mentally unwell, paranoid, and physically unwell, and that can lead to suicides and violence against other members of the public."
Mr Middleton said he was aware of one suicide attempt this week by a teenager in Manurewa who took synthetic cannabis.
Mr Bush said offenders demanding synthetic cannabis products committed seven robberies in Otago/Southland recently, using weapons ranging from knives to firearms.
In a Dunedin case, synthetic cannabis was "the motivating factor in the case of a 17-year-old who recently appeared in court ... following a crime spree that involved breaking into 27 different vehicles".
It is illegal to sell synthetic cannabis to anyone under 18, or to discount its price, but a police "sting" disclosed in Christchurch yesterday caught two dairies selling it to a 17-year-old and one offering two packets for $25.
In Manurewa, local board member Simeon Brown photographed a sign outside the High Zone shop last week advertising, "Opening special: legal highs for $15 only." Shop owner William ("Chris") Kashyap did not return Herald calls last night.
The police crackdown comes amid mounting concerns from health services, including after a Timaru 14-year-old had a heart attack after using synthetic cannabis.
Dr Leo Schep of the National Poisons Centre in Dunedin said two people in the Bay of Plenty, two in Christchurch and one in Nelson had been hospitalised with acute kidney injuries. "Two required intensive care. The other three were admitted to renal wards."
A parliamentary committee opened hearings yesterday on a bill that would shift the onus of proof on to anyone selling psychoactive substances to prove that their products are safe. If passed, the bill is expected to become law in August.
Mr Brown and Manurewa Local Board chairwoman Angela Dalton urged the committee by videoconference to give local communities the same kind of control over shops selling psychoactive substances that they have recently been given over liquor stores.
Manurewa youth group Warriors of Change is organising a protest outside the new High Zone shop at 11am on Saturday.
However, industry spokesman Grant Hall said closing stores would be ineffective because psychoactive substances were freely available on the internet.
He said "tens of thousands of packets" of synthetic cannabis were being sold in New Zealand each week, with sales valued at $25 million to $35 million a year, yet only 15 people had been admitted to hospital emergency departments in the last three months after consuming the product.
In the same period, he said, 651 people were admitted with problems caused by alcohol and other drugs.
"Alcohol kills 17 New Zealanders a day. Synthetic cannabis has never caused any fatality in NZ," he said. "It has been around in NZ since 2001 and it's only in the last few months that we are hearing a cacophony of noise that seems to us to be based on rumour and scaremongering."
Mr Hall said health problems arising from synthetic cannabis were caused, in every instance he had investigated, by people not following directions on how to consume the products safely or consuming alcohol at the same time.
