Police in China have been known to extend a detention period by starting an investigation into a new offence.
It is also possible Gardner has already been charged and authorities have not yet made a public announcement.
Mfat said a diplomat had visited Gardner in detention.
Chinese authorities alleged Gardner tried smuggling 30kg of methamphetamine, out of China.
If found guilty, New Zealand-born Gardner, who lived in Sydney, could face 15 years to life in jail or the death penalty.
Gardner's father Russell Gardner told the Herald on Sunday his son was "doing okay" despite possibly facing death by firing squad.
"We're managing, we're pulling together. We're still a close family. The Chinese are treating him okay."
Russell Gardner said his son had never been in serious trouble before, but didn't want to speculate on his guilt.
Guangzhou Customs told the Sydney Morning Herald Peter Gardner was the owner of two drug-filled bags, the zippers of which were sealed with super glue.
Guangzhou Customs alleged that when the bags were opened, officials found 60 vacuum-sealed bags containing 30kg of meth.
It was reportedly the largest single haul of ice headed overseas Guangzhou Customs ever seized.