Underwater property is not new phenomena, it happened before in HK properties severely back in 1997, and just years ago in US property.
Bank generally do not want to foreclose the "underwater" property.
As long as borrower service the loan, they are happy to status qou generally.
As in an "underwater" property, borrowers can be best to walk away the property, the let banks foreclose it.
Eg. A property previously 800k, with monthly loan 4K,
Now the property only worth 500k, which now 3k (just simply a number)
If the borrower willing to service the loan at 4K consistently, banks are happy to see it instead try to tell the client to top up the collateral margin which might lead to foreclosure, whereby bank only can recover 500K instead of 800K loan amount. A 300K NPL potential incurred, which in massive scale (as banks could have ten or hundred thousand of similar borrower) which could bring down a bank as well.
But if the borrowers still paying 4k every month, NPL won't incur in the bank book.
In this kind of situation. borrower can choose to walk away the property, and save 1K every month which technically is "burned" away in an "underwater property, the borrower is paying at the valuation of 800k.
Or in other word, the borrowers are paying 800k that only worth 500k currently.
There is reason why bank reluctantly to give full valuation on properties in recent years, as it gives margin of safety for the collateral issue for the bank besides the valuation issue.
The difference between USA and malaysia is in the states once foreclose the debtor don't have to pay the underwater loss, they can walk away without further liabilities. In malaysia, debtor have to settle the difference after foreclose, otherwise the bank will sue for bankruptcy!