Mr Phillips said the man had the "jar of chemicals", understood to be a liquid, in a bathroom at the mall.
Numerous police, firefighters and St John vehicles were seen at the shopping centre on the corner of Great South Rd and Manukau Station Rd.
Fire Service hazardous material crews decontaminated personnel and controlled the chemical, said Mr Phillips.
Westfield spokeswoman Debra McGhie said the mall was not evacuated and all shops were open and operating again.
An NZME. News Service reporter at the scene said emergency services had left and the bathrooms were completely reopened.
United Sweets employee Sohang Patel said all the action wrapped up about 3pm.
His store is opposite the entrance to the bathrooms where the incident occurred.
No one was allowed to enter after a security guard closed off the entrance, he said.
Ambulance officers and firefighters arrived shortly after.
"We were asking the security guards and wondering what was going on," he said.
There was a back entrance to the bathrooms from the road, he said.
He saw nothing until the bathroom was reopened about 20 minutes ago.
A police spokeswoman said the man in the bathroom was understood to be "self-administering drugs".
"The mall wasn't closed or evacuated, there is no risk to the public and it's business as usual now," she said.
Ms McGhie said the man was found by a staff member, who alerted emergency services.
Mall patrolwoman Nancy Latu said she was cleaning the woman's bathroom next door when she a heard noise.
Another cleaner found the man after knocking on the door of the cubicle to get him out, she said.
They were told to clear the area by the security guard, Ms Latu said.
It smelled very strong, she said.
"For me it was almost like when you enter a hospital," she said.
"I came outside and saw an ambulance officer run outside and try to breathe."
People working at the mall at the time told the New Zealand Herald they saw police officers and security staff gathered around a mall bathroom, but the event caused no panic or disruptions.
- With additional reporting from NZME. News Service