You're biggest danger there in april is the mosquitoes and other insects, especially if you are walking around with a torch...they can be overwhelming at that time of the year. Watch your ears and eyes.....a blister beatle in your eye can be very nasty. Seriously consider wearing long loose pants, long sleaved shirt and perhaps a flyscreen headshield at night if you're not used to it.
You do need to be crocodile aware, some big crocs live in the dam and will often lie on the dam wall and edges at night to try and cool off. Both salties and freshies live in the dam and on the surrounding floodplain. They could be anywhere in this area....including small isolated drying pools on the floodplain. If the croc is on land you're probably quite safe and they seem to be more scared of you. Keep your wits about you and don't turn your back when close to the water. With caution you are ok walking back and forward across the dam wall at night and you should see quite a bit of stuff...it's a great spot; although I haven't been there for years now. On many occassions I have been there later during the dry and stopped the car for one water python and seen another 2 or 3 crossing at the same time in range of the headlights!
During april things are still quite spread out although there will be some movement back from the floodplain to the main dam. Used to be lots of big panoptes there during the day, before the toads. Just before dusk keep an eye out for Golden tree Snakes in any of the trees near the dam, very common. There's a good chance of seeing long necked turtles crossing the road as well. Just on dusk is when you see the peak activity time for keelbacks moving around. After dark you should start seeing the water pythons although not in great numbers at that time of the year. You may also see Slatey greys and macleays, and file snakes on the road and watered edges. Back in the verging monsoon and pandanus forest you could come across adders (although they have taken a huge hit from the toads along with king browns), carpets and childrens and often frillys during the day...lots of northern water dragons as well.
The drive out across the marrakai plains on either side of the adelaide river can also be very productive. Watch out for road trains!!!
Also keep an eye out for pole 207 on the eastern side of the adelaide river, it leans and is on a slight bend in the road. Of special significance to me. Wet season 1997 following on from Cyclone Rachel I tied my troopie to that pole at 10 pm one night. The old troopy stopped in 1.4m of water. At that stage the nearest land was 1.7 km away on the edge of the floodplain. We had to abandon ship and tied it up to the pole for safekeeping overnight although I don't think the lean of the pole has anything to do with us!
Enjoy your time around Fogg dam and the Adelaide River..a great place to spend a few days and make sure you put up a post with pics when you return.