So it will often depend on who is doing to medical assessing - the insurers tend to outsource the assessment process to third-party medical risk assessment companies, so there's like four companies doing all the online medical assessments for all the travel insurers in Australia. They tend to be linked to who the underwriter is, so many of the travel insurance brands underwritten by Lloyd's have Healix doing the medical assessments, a lot of the Allianz underwritten brands have the same medical screener, etc. Having the same screening company means they tend to cover and exclude basically the same stuff unless an insurer has specifically requested to allow it.
So while a lot of brands do cover cancer, the questions the will ask about the cancer will often prevent coverage, particularly if you're undergoing chemo at the time.
If there are some that allow it, the tips I would offer for finding them would be to look at a list of all the travel insurance brands in Australia and who their underwriter is. That *might* help weed out which have different screening companies as well, which tend not to be disclosed anywhere. The other thing would be to look at a list of companies which have online screening forms.
The mods have suggested that I'm not allowed to post links because I have worked in travel, but if you google travel insurance underwriter comparison and travel insurance pre-existing medical conditions comparison, you should be able to find sites that will help.
An alternative option worth looking at would be specialist medical travel insurer, AllClear. My understanding is that they don't automatically exclude any conditions, although the premiums can be pricey af. I tend to recommend older folks with heart conditions check them out, but they might be able to help here too.
