A major apartment builder in Melbourne has suddenly stopped operating, vacating its offices and abandoning building sites, in an impending second collapse linked to Watersun Homes.

The Victorian arm of Watersun Homes, WSH Group, was recently placed into voluntary administration with $20 million in debts.

Now another collapse could be imminent, leaving further tradespeople with huge unpaid bills.

Watersun Construction, a separate company associated with a number of commercial projects in Melbourne, has seemingly come to a halt.