Water leak going out of control

[Brandt Tingen]

Hi,  My tenant's water usage is normally 60 - 500 gallons per day.  I just looked at my bill and the water usage has been 3,000 - 7,000 gallons over the last few days.  I called the tenant immediately and asked what was going on and they said they had re-installed a new flapper but it was leaking.  Is it possible for a toilet to overflow this much, or does this much water have to be a burst pipe?  We have had very cold weather the last week and a half.  I asked the tenant what day they tried to repair the toilet and the day after is when the water usage spiked.  I have sent the tenant a letter reaffirming that they are to make no self-repairs under any circumstances, but looking for some ideas for now.  

Also these tenants are home all of the time, so if anyone recommends bomb-proof and leak proof toilet guts or kitchen sink faucets and parts, I would gladly take suggestions.  I am even open to installing commercial grade equipment.