CFA crews extinguished a flaming pile of tyres and a gas cylinder in Lyndhurst.
A security patrol spotted the flames about 3.30am on Friday 18 May and notified firefighters.
They were concerned about the gas bottle in particular, as its blast radius could have been up to 500 metres.
Greater Dandenong CIU is investigating the deliberately-lit blaze in Glasscocks Road.
They said the block was being used as an unofficial dumping ground.
Call Crime Stoppers on 1800 333 000 with any information.
