I'd also check out the Australian Human Rights Commission website section on insurance http://www.hreoc.gov.au/disability_rights/insurance.htm in relation to disability discrimination. Cases and conciliations seem to confirm the below.
My understanding is that most insurance including TPI and income protection won't cover pre existing medical conditions but should cover unrelated future conditions and situations, ie being hit by the proverbial bus.
But one has to be honest about everything when asked, say in their questionnaire, ie past any condition to ensure the insurance is valid.
However some insurance cover will still cover cancer. For example a past work place I worked at where their superannuation also offered TPI and income protection they actually did not ask questions about pre existing conditions and did cover claims. Their rationale was that the organisation was large with 14,000 employees of working age (15-65) which statistically this population in general has a low level of disability (10%) and thus the risk of covering an employee with a pre existing condition was was spread over a large range of low risk employees. There was also a 3 month wait for income protection to kick in and many conditions although serious are too short term ie heart bypass, minor heart attack, even major surgery without ongoing complications or treatment where most are back at work after 3 months.
Cheers DJ