Fire Service safety inspectors this morning concluded the blaze in Onehunga was suspicious after eliminating possible electrical and accidental causes.
The investigation has now been handed to police.
Store managing director David Grove said his family and staff were distressed by the fire.
"We are all obviously stunned at the report's findings, and can only reiterate that we are so thankful for the quick actions of our staff and the New Zealand Fire Service," Grove Mitre 10 managing director David Grove said.
"We will be stepping up our security procedures across the business to continue to safeguard our staff and employees."
The fire took place in the Mitre 10 expo centre, across the road from the home and trade retail outlet which remains open.
Mitre 10 chief executive Craig Wilson said he was shocked at the findings.
"We are now focusing our attention on assisting the relevant authorities with their investigations and providing full support to the Grove Mitre 10 business."
Manukau chief fire officer Larry Cocker said fire safety officers concluded this morning that the blaze at the Onehunga Mitre 10 was suspicious.
"Following investigations and interviews with people present at the time we have eliminated all of the accidental causes," Mr Cocker told NZPA today.
"We have now handed the investigation over to police." Mr Cocker said he wasn't sure where in the store the fire originated. He was not aware of any accelerant having been used.
The Mitre 10 hardware exploded in flames late on Monday afternoon. Within minutes it was a flaming inferno as gas bottles, paint, kerosene, methylated spirits and numerous other hardware lines fed the ferocious fire which sent up a huge pall of black smoke.
Around 80 firefighters and more than 20 fire appliances battled the blaze for three hours.
Mr Dickson said the building had a very high fire loading because of the contents of the hardware shop and there were no sprinklers.
Fire engineers have been constantly pushing for sprinkler systems in such buildings.
Power was cut to the area and several roads nearby, including the motorway to the airport, were closed.
