Unless someone with 5 million per month buys and flips, I don't know how that is possible to have 2-5 properties, as rents here do not sustain paying off a mortgage. The money is from within Indonesia and property prices are going up/or have been but I just don't see the incomes of most people keeping up including buying and renting to pay off a 2nd or 5th mortgage on 5 or 10 million per month. There is definitely a lot of money floating but most salaries are relatively small.
I guess if a person looks at the extent of corruption right through the system, then this can account for a lot of money going into the housing market and historically Indonesians have put their money in property. I am not sure where the ceiling could be for property prices, but most rents do not work to pay off mortgages, and of course their are regional etc variances. I guess a lot will have to do what direction the country takes in the future. I know house prices in parts of Spain eg. were way down a few years ago and of course in the US after 2008.
Of course there are many Indonesians who are also able to pull themselves up, through hard work and sacrifice.