Rob,
I know where you mean. I'll take a wander up there this weekend for a look see of I get off the rig on Friday 👍
The Ikan bulan (Indo pacific tarpon) are very easy to catch on the fly. I tend to outfish the locals 10 to 1 with their spinning gear 
Did you ever investigate any of the larger lakes/dams for giant snakehead?
I'm assuming you were based at the hanaffi?
Steven.
We lived in Mumong and then Panaga. I was but a teenager so my fishing was limited to where I could cycle.
The sungi's on the golf course in Panaga were my main spots, fishing for things I could put in my tank...Clown Loach, Tiger Barbs, 3 spot Gourami's, Glassfish, climbing Perch, Bumblebee Gobies, various Barbs etc
I used to catch them on tiny hooks, a little bit of bacon rind and a "tenkara" rod.
However, I started to notice big swirls and splashes under the overhanging trees and discovered that there were Datnoids (Sumatran Tiger Fish) in them. Superb fun on a fly rod. Grow to about 6lb and look a bit like an angry Perch crossed with a Barramundi! I only had an old 7 weight rod back then but a 5 or 6 weight would be ideal.
I also used to fish the estuary at Seria, both on the seaward side and the lagoon side. Mostly had catfish but I regularly bumped into a Chinese gentleman that fly fished it. He mostly had Barramundi, Mangrove Snapper or Mullet.
Talking of Mullet...when the sea is flat calm, its well worth walking with polaroids and spotting shoals of Mullet. Incredibly spooky, but they will take a fly. Its the closest I ever got to Bonefish fishing
Up by the Anduki airfield was most productive.
There were a couple of ponds up the Labi road as well, stocked with Carp. I understand they were part of a Government experiment to see if carp could be a food source, letting them swim around the padi fields until ready for harvesting. I never got to fish them but I had heard there were carp around the 30lb mark.
As for Snakeheads, I had never seen or heard of them when I lived there (30+ years ago) but you could buy them in certain stores in Kuala Belait and Seria. It would not surprise me if they found their way into the waterways. The other "big" fish that the Chinese bought were Arawana. I remember seeing fish 3 feet long curled up in 2 foot tanks in the shops!! There MUST be some of them in the sungais.
