As a REALTOR® I am asked all the time what is MPAC and what does it stand for? It stands for The Municipal Property Assessment Corporation and is an independent, not-for-profit corporation funded by all Ontario municipalities, accountable to the Province, municipalities and property taxpayers through a board of directors.
MPAC will assess over 5 million properties every 4 years worth roughly $2 trillion dollars. Basically they take sales data and compare it on the basis of lot size, location of your home, the area that you live in and the quality of construction.
Remember this is not a letter of Opinion or a market analysis or for that matter based on anything close to what the a seller could receive on www.realtor.ca or the open market. I find it interesting that they take into consideration the quality of construction, how? Google 360 camera? Just joking I suspect its based on building permits that are given at the local municipal level.
Bottom line if you feel you are paying to much in property tax, especially if you have a home that does not have a comparable ie a country home speak with your REALTOR® and get a real assessment