Hey mate.......not sure about the rain thing. But my first guess without more information ( CEL come on ??)
1st....I would say the the IAC valve is stuck open.
2nd....Bad wiring or connections with moisture getting inside. Spray all connections with WD40.
3rd... Stop driving the ute thru deep water and get a tinny !!!
Cheers !!!
JzJzz and the Man from Down Under....Time to head to the counter lunch and have a coldie!!