If you remember to open them before it is an issue then probably no big deal...but I know I would forget sooner or later.
If you look at the first 10-12 inches of each blade you will see that they are round...no aerofoil shape at all...the only air going through the cowl at idle is what will be sucked in the front by the venturi effect of air going past the cowl flaps and sucking air out.
I tend to think it doesn't get cold enough in Oz to be bothered given the risks of eventually forgetting them and taking off with closed cowl flaps.