If you can extend your stay in Australia on tourist visa, deepens on your circumstances. If you currently have ETA and eVisitor, you are allowed  to apply for 600 onshore. However there are restrictions. If you are already in Australia and hold a valid Electronic Travel Authority visa (subclass 601) you can extend your stay by applying for another visa, such as a Visitor visa (subclass 600). An eVisitor (subclass 651) cannot be extended.
You can not extend your travel visa  if you have  has been in Australia cumulatively 12 months on visitor visa.
You can not extend your travel visa  if you are on working holiday visa.
You can not extend your travel visa if you are on student visa and want to continue study.
You can not extend your travel visa if you have not if you have no further stay condition.