Hi I was wondering if I may have been discriminated against…

Hi I was wondering if I may have been discriminated against in the workplace. I had a "formal chat" regarding my apparent disregard to safety in the workplace yesterday with my boss. I work as a diesel mechanic on a mine site. I have had a couple of minor reported incidents (cracked a windscreen on a Cat 994 loader with an overhead crane whilst installing an implement pump, and had a minor injury to my shoulder which required physio but no time off work whilst trying to install a brake cooler tube on a cat 793 dump truck). He also cited 2 unreported incidents, firstly saying I had been seen to drive the forklift too fast (I can only think of one example of this, and was chatted at the time), the other being I had gone forwards instead of backwards with the forklift while towing a track off a Cat D10T dozer (I was not proud of this, and accept this was innappropriate).
The thing that concerns me is that this "formal chat" (ie verbal warning) happened to occur at the same time as he announced that my pay had been reviewed. I am currently near the end of a dual trade apprenticeship, and have finished my tafe studies and have my parchment to say I am qualified, however am still being payed 4th year apprentice wages despite being verbally told that I would get full tradesman rates with this review. I was very happy where I am working, I know feel like I have been lied to yet again (when I took this job 3 and a half years ago, I asked if I finished early, would I get signed off early. HR said that there was no reason this could not happen, as I already had one trade, so was not a true apprentice.) I signed with the company for a four year ffixed term, however this was under the impression that those four years would not all be at apprentice rates. My questions are-can I leave this job now as I am qualified, and can my employer continue to pay me this rate even though I am qualified. I feel that they will now use this verbal warning as an excuse to keep paying me this rate until the end of the four year term (june 2011) Thankyou.