If you're an Origin Energy customer, simply call them (13 24 61) and ask for 10% of your electricity to be Green and they will ensure it is with no additional charge. To be clear you are not getting 10% off, you are simply getting 10% of what you are already paying for as power from a renewable source (http://www.greenpower.gov.au/Homes/) instead of coal etc. which most companies will charge you additional for. Origin has other options starting from 25% GreenPower (for an extra $1 a week), but if you want to make a small difference without paying anything extra simply ask for the 10% GreenPower.
I came across this at the start of the year when I was reviewing who to use as my Electricity provider - the Origin consultant mentioned it when I asked about their GreenPower options and I confirmed it again when I spoke to them today, but you will not find any mention of it on their website (the link I've provided just shows you the GreenPower options you can pay for).
It is available for all Origin electricity customers.
Note, when you receive your next bill after adding it, it will say "Green Electricity" in the top right but it won't mention the percentage anywhere (I called and confirmed that 10% was being applied though).
Once again I'm not recommending for or against using Origin for your electricity (do your own research/comparisons) but if you are an Origin customer it's a free thing to do to make a small difference (ideally purchase a higher percentage of GreenPower but that's a discussion for a different forum).