I think most teachers (if not all) definitely mark more harshly for internals than how HSC marks mark the HSC exams, for 2 main reasons.
1) By teachers marking harshly, it prepares students to be conscientious of the small details, which can differentiate a low Band 6 student from a high Band 6 student (sig.figs, rounding, diagrams, technical language, etc)
2) Internals are supposed to differentiate students within a cohort, so it may seem like they are trying to take marks away rather than give you marks, but a strong internal rank, although may be backed up by a poor raw mark, will be very beneficial for moderation, especially if the cohort can perform very well in external exams.