You can't trade the spi using Commsec or Mac CFDs (Although Macquarie has promised an index cfd).
$25 a point means exactly that. Today ranged about 50 points so you could have lost $1250 per contract if you got it wrong. Some of us use CFD indexes which can be as low as $1 per point (CMC Aussie200 CFD) which is a much cheaper and fun way to learn.