The wording in both Q8 and Q18 is key.
Q8 - In my opinion is legislation. Yes in the past there was a question similar and the answer was common law, but the wording was not the same. Recently there has been enough changes to suggest legislation is certainly the best way to protect human rights. The question was 'human rights are best protected in Australia WHEN THEY ARE'.. Not, 'HOW are they best protected' which was a past question. There is a difference in my opinion. Also, are they really 'incorporated' into the common law?
I've read the argument that legislation can be changed, so it is not very protective, but I would counter that argument with the fact common law can be expanded or reduced by legislation passed by Parliament also.. Someone has already provided information from the humanrights.gov.au site which provides "human rights reflected in the common law lack adequate protection".. There have been similar questions in past trials and answers have all been statute so if NESA have the answer as C I would suggest there will be some complaints lodged.
Q18 - Again, the wording is key. The question asks 'which of the following CAN be used as a partial defence?' Now, whilst you are all correct in saying insanity, necessity and self defence are all in fact complete defences - so by process of elimination accident should be the answer, as mentioned this is not a defence. Implying that NESA may be referring to 'mistake' is a long bow to draw in my opinion anyways.
The answer is D - Self defence, which CAN be used as a partial defence.