Officially, except for Victoria and Albert, there are no restaurants with a formal dress code. Park Casual is the accepted mode of dress. As a general rule this tends to mean no swimsuits and, at the non-park restaurants, no men with "muscle shirts".
Certain restaurants (such as CG, Narcoossee's and Bistro de Paris), which put on an upscale showing, say that "Business Casual" is recommended, but I have never heard of people neatly dressed with T-Shirt, jeans shorts and sandals being turned away.
I think this is especially true as there are people who are coming directly from a park (or, in the case of Bistro are in a park) who cannot change. Let's say someone without a car was staying at AKL, was at MK, and had a reservation at Narcoossee's. Two choices. One is to get the yacht from MK to GF, which lets them off almost at the door to Narcoossee's - Time about 15 minutes. Otherwise Otherwise bus back to AKL, get to room to change (possibly showering first), bus back to MK and the boat to GF - Time over an hour.