Thanks for your time, Steve"
I'll look to do a full army comparison on this some time but it's certainly not all doom and gloom for Tau against Necrons. First of all, their main anti-tank laughs at AV13 and if you don't kill it once it goes through, the army laughs at AV11 in general, particularly open-topped. Blacksun filters are certainly your best bet here and they are something you should probably be taking on each of your Broadside units these days - annoying but there it is.
A single large Scarab farm is great for Tau really - Necron anti-infantry is great up close - not so far away. If you can keep your Pathfinders safe until they are well within spotting distance of the Scarabs and strip their cover, Crisis Suits with missile pods and/or plasma guns will tear a hole right through them and you do have some good squads for blocking delaying in Piranhas, Kroot, Devilfish and Drones. Use those to stop the Scarabs from getting to you and you should be okay. Remember Markerlights can also allow a unit to ignore night fight so you don't have to expose your Crisis Suits here.
In the end, Tau v Necrons are like most Tau match-ups - if you can apply your shooting as much as possible you'll do well. Necrons can clear your infantry out pretty well if they get close and can restrict ranged shooting with Nightfight but if you can overcome that with blocking and markerlights/blacksunfilters you'll do okay but pushing back into midfield will always be a problem even if you do protect your Crisis & Broadsides.