"It's possible someone may have climbed on to the roof of the building, pulled the container up with the twine and smashed the skylight to start the fire," Mr Alexander said.
"The skylight was destroyed in the fire, so we have no way of verifying that, but it is a possibility."
There was no sign of forced entry into the building.
Police would like to hear from anyone who recognised the container or may have noticed someone acting suspiciously on Friday.
"There would have been some traffic in the area at that time of night, so we are keen to speak to anyone who saw any vehicles or people that seemed out of the ordinary in the Onekawa industrial area," Mr Alexander said.
Petrol stations were also being asked to contact police to see if staff had noticed anyone filling the container.
-Anyone with information is asked to phone Mr Alexander at HB police on 06 211 3791 or anonymously on Crimestoppers 0800 555 111.