Get a stump remover dude in. He has a machine that munches it to pulp and he can dig down a bit and also remove the main roots. They're in the phone book or your local pages. I think they're about $120 depending on how big the tree stump and root system is. Takes about an hour or so. He can take the pulp or better still keep it and put it on your garden 'cos it's excellent mulch.
Either that or you've got a lot of hard yakka ahead of you with a 60cm stump. A lot of hard work!