Whilst Freshwater Snake or Keelback (Tropidonophis mairii subsp. mairii) is the first thing to come to mind when I see this and the most likely suspect, based on pattern, build and location, one cannot discount Rough-scaled Snake (Tropidechis carinatus), which are also found in the area. The quality of the photo unfortunately doesn't allow for a positive ID, therefore it's best to leave this as a suggestion only in my opinion.