There is a sign saying "Government Land: no unlawful occupation". I contacted the Tuen Mun Lands Department in March 2011, and was asked to send a map and details. However, since then they have ignored my emails and telephone calls.
It seems likely that the site is being illegally occupied under a "destroy first, build later" strategy. This is unacceptable because the land has other possible uses.
Perhaps the Tuen Mun Lands Department knows more than I do.