It just so happens I am working on a post production project and had to remove induced hum (power line induced 60Hz) from a voice over.
 
Just for curiosity sake.....
 
Those of you who live in countries where there is 220VAC @ 50Hz, when you find any instances of line induced hum, is it at 50Hz? It's an assumption I want to confirm.

Yep, it's 50Hz or harmonics of 50Hz. Which are most commonly strongest at 100Hz with higher harmonics depending on how 'leaky' the circuit that's inducing the hum is and how much gain it's applying.

Waves give the option of choosing 50 or 60Hz simulated mains hum on a lot of their hardware emulations. Not because the emulations hum very much, but because if you use a bunch of them and they emulate 60Hz mains while you've got e.g. guitar amp hum at 50Hz and multiples thereof the result can be quite nasty.

Edited to add - while in the UK the voltage might vary a little, e.g. where I am it's usually around 237V but can drop to 232 or go up to around 245 the AC frequency is rock solid at 50Hz. The voltage is theoretically 220V across the EU, but with a big enough margin of error allowed to mean when that standard was adopted no country had to change the voltage it already supplied. The UK being nominally 240V at the time.

Our mains voltage here is also 240 V not 220V and yes 50Hz here in Australia.  So any hum is related to that frequency.
 
I usually use some form of noise reduction software for something like this.  Sample the hum on its own and then let the software remove it.  It usually does perfectly.  The more constant the interference the better for noise reduction systems to remove.  Or a steep scientific filter at 50 Hz like many dB down at that frequency and back up at 49 and 51 Hz.  But you do need to filter out harmonics too at all the other related frequencies etc..x1..x3...x3 etc..
It is more like 100Hz that you are hearing too if you do get severe forms of hum induction taking place. 
 
 
The 50 Hz frequency is better for many reasons.  One being 25 frames of video fitting exactly into that frequency. Frames are exactly 40 mS wide.  Unlike the silly frame rates that exist in the US.

That's what I needed to know. Thanks for the info tlw and Jeff. Much appreciated