I finally found this place after taking a city bus on a Monday morning. The opening hours are 8am to 6pm, but closed on Monday. But I noticed the gate was unlocked so let myself in and walked around a bit. It has a park-like feel with lots of big trees and of course the marshy area. Lots of birds about. You can easily walk back to downtown KK by turning left at the exit and walking up the hill. Just keep following the road and after about 2 miles you're in downtown Kota Kinabalu. It's a nice walk.