It depends on the measures that are taken. As per the Board of Studies site, one or more of the following can apply:
— reduced marks for all or part of the examination
— zero marks for part or all of the examination
— an interview with a ‘malpractice’ panel at the Board of Studies
— loss of one or more courses towards the HSC award
— damage to the student’s ability to apply for entry to TAFE or university courses or scholarships.
For example, in the situation you mention, a half-yearly is of reasonable weight. If a student were to receive 0 in an Advanced English half-yearly, it could potentially impact their overall result to a great degree. Not to mention that it is very likely or certain, if you do only 2 units of English, that this overall result will contribute to your ATAR. Therefore, 0 in an Advanced English half-yearly can ultimately affect your ATAR as well.
In simpler words, malpractice is not something you should really be considering the consequences of, since consequences are only relevant if you are thinking about conducting malpractice in itself.