If it is solid plaster (as it sounds or you could just whack on a new sheet of gyprock) you might find the damp in the wall will try and ooze its way out through the new plaster and make stains. If there's any salt it won't stick, but with just a water leak you should be ok.
You can make the stuff to render the wall out of hydrated lime and hardwall plaster (mix as per packet directions), Bunnings sells both but hidden in a deep dark corner and their staff don't seem to know about it. Its about $10ish for a 20kg bag and they probably don't sell smaller bags. If you want the equivalent product in a premix that is easier to work with, cornice adhesive has loosely the same properties, costs quite a lot more per kilo but comes in smaller bags if you don't need so much.
Top coat won't dry any time soon on a damp wall and is very soft. Plaster and lime (this includes cornice adhesive) will dry underwater (I'm not kidding) and is hard and smooth - almost glossy - but you have to tool it smooth with a clean, flat metal trowel while it is going off (which is an art form in itself), you can't sand it. You don't want to do a deep coat of any kind of plastery stuff as it is very hard to work with on something more than about 1/2cm deep. Top coat is designed for very very shallow joins between plasterboard and will crack if you put it on more than about 1mm thick, anything plaster based can be put on as deep as you want and won't crack, it just gets hard to work with when it is deep. Get some brickie's mortar on there first to render the wall to the right depth if you need to, and if there's issues with it sticking add some bonding agent to the mortar.
Yes, I've done a LOT of hardwall plaster repairs in the last few years. Its so hard to get the damn stuff flat and smooth we just pay an old (young bucks usually don't know solid plastering) ex-tradie to do any larger areas. He makes it look easy.
I'd still put a blower on that wall for a day or so first, whatever you do.
