Engineering / Mathematics Graduate here.
I get what you're saying. You're probably studying harder than you ever have your entire life. You probably asked this question because your afraid that your workload is only going to increase later this year and it's already bad enough as it is. You're biggest fear is probably wondering when will the "end" finally come where you no longer need to worry about assignments, exams, homework etc.
If this is you then I have some good news for you, it gets better after the HSC.
To answer your question, is engineering harder than the HSC, absolutely but not in the way that you think. What you need to understand is that the HSC is your first ever experience of a major event that you have to prepare for with real world consequences. From here on in you get much better at dealing with this.
Is the content harder? Yeah sure it is, but you have a lot more tools to help you out. There a lot of advantages that you will have e.g. you will only do 4 subjects every six months and most of them don't need you to remember what you did the year before. You won't be in class for 7 hours a day 5 days a week. You may have the odd day or two with terrible 12 hours worth of lectures but you will rarely have that more than once in your degree.
Engineering is definitely not an easy thing to do, but I would argue that it's easier for an engineering student to go through an engineering degree than for a HSC student to go through the HSC. It's not a walk in the park but you won't have that sense of dread a lot of HSC student's get this time of year.
Take things one step at a time, Good luck