So this answer assumes that you are just interested in getting better at integration itself, rather than HSC integration in particular. If you are already on top of the integrals in past papers and the standard HSC textbooks, you will be absolutely fine for HSC integration. If you want to improve your integration in general, then there are many sources. (But do not use any higher level techniques you learn from these books in HSC examinations.)
Some books you might find of use if you haven't checked them out already:
Inside interesting integrals - Nahin
Irresistable integrals - Boros,Moll
You will almost certainly learn some new tricks from these books, and get some nice integrals to practice them on. Keep in mind that you might need some mathematical maturity beyond the high school level for certain topics. The Nahin book is newer, and its introduction references the differences in style/philosophy with the Boros/Moll classic. You might find one more to your taste than the other, but the content is not identical so check out both.
The handbook of integration - Zwillinger
Kinda different in style, and maybe less useful for learning than the above two. More a book to have on your desk to have a checklist of things to try when it comes to integration etc. In other words, a better reference book than a textbook.
A treatise on the integral calculus - Edwards
Quite old fashioned, but lots of problems you can mine from here on integration and applications of calculus etc.
Once you have mastered some new techniques, you can find new problems to practice them on at math.stackexchange (questions with tags related to integration), brilliant.org (past integration contests), artofproblemsolving maybe as well etc. If you are driven I am sure you will find many more.