Hi, it appears you may have a legitimate claim under section 54(1) of the Australian Consumer Law. The section relates to statutory consumer guarantees. S 54(1) imposes strict liability on suppliers of goods, and they will be liable if the goods supplied are not of acceptable quality (even if the supplier was in no way to blame for the fault). So if the manufacturer is the one that supplied the benchtop to you, it would be easier to take them to QCAT. Under the consumer guarantees, the goods (in this case your benchtop) must be:
(a) fit for all the purposes for which they are sold;
(b) acceptable in appearance and finish;
(c) free from defects;
(d) safe; and
You would argue that the cracking of a benchtop and the failure of a join within only 5 years of normal use, means that all of these requirements were not met. The fact that you rejected an offer of repair is not fatal to your case, since you would argue that the offer of repair would not cure the requirements above, since the repairer could not restore it to an appropriate state.
I would think you had a good arguable case against the manufacturer, and given the low cost of accessing the QCAT system, definitely worth an attempt. If you succeed at QCAT, they can impose a civil penalty on the supplier, and/or order damages or compensation to be paid to you. Often, the very act of lodging a matter with QCAT will 'encourage' the supplier to settle the case. Hope this is of assistance. Regards, Jared