I don't keep monitors but Perenties are native to the hot arid areas of northern Australia and while they survive the cold nights of inland desert areas by digging burrows they have evolved to live in a climate where year round daytime temperatures are always high. Putting one in an outdoor cage in Victoria is animal cruelty and it will not survive.
I suggest you read one of the good books available on the subject and do a bit of research.
Perhaps one of the experienced monitor keepers on this site can advise on cool climate monitors?
A lace monitor could work if it had a nice, insulated heat box. They're pretty similar, though lacies are more of an escape artist due to their arboreal nature.