Hi,
Whilst I am not a fan of private health insurance, I have it just in case (and to avoid problems with lifetime loadings & medicare levy surcharges).
I see that there are regularly bonuses/promotions. eg. Free month, or free $100/$200 gift card etc. . Currently I believe the best is $200 gift voucher from NIB - https://www.ozbargain.com.au/node/268750
I believe that most health insurance providers will transfer your history over from your previous health insurance provider so that you don't have to go through waiting periods again etc.
So is there any problem in changing private health insurance providers regularly (eg. every three months) to take advantage of a promotion? Obviously I would be changing to comparable plans each time. I can imagine it makes your tax return a tad bit more difficult - but I believe the information all gets pre-filled in anyway.
Once you serve waiting periods you don't need to serve them again.