Be prepared to get very additive to this. Once you go in you may never return. I'm still at it and it's been a whole year now.
I don't know what the laws are like in your country, however, in the UK we need a special license to be able to go anywhere near the nest, so best checking.
Anyway, whether there's a law or not, the Kingfisher is extremely sensitive to humans near their nest. Don't go near it for any length of time and don't stay long unless you're hidden. They will abandon their young in the nest if they feel threatened.
You'll get loads of great advice from seasoned pros on here no doubt, however, I'd ask myself what pics you want to get?
If you want food passes, mating and interaction then it's the nest site for that sort of thing. It's likely to be all over by now so you may have missed it. If it's just perched on a stick 5-10m away then you'll need some sort of baiting pool and expect it to be a long process for the kingy to fish from it.
I set up a pool during the winter with the river flooding next to the river in an overspill stream. I just made a pool in the stream out of dirt and leaves, put minnows in it by way of trapping them and the kingfisher took about a week to regularly fish from it.
A 400mm lens will mean you'll need to be within about 5-7m away to get Full frame shots with minimal crops for composition. Anything further away and you reduce that. I can get about 15ft from a king without the need for a hide but that's only when you get to know their spots.
They will tolerate you being there if they fly close to you, however, don't expect to just wander up to one with a long lens.
It's a weird time to get into it as they are normally in the nest on eggs, so in the next month or so they'll have fledging young to chase down lol.
Honestly though, be prepared to sleep, eat and breathe this stuff for a while as you'll need it