Mate, forget dealing with the shop, its getting you nowhere. It's time to make a phone call....

Western Australia

Minister: Hon Sheila McHale, MLA, Minister for Consumer Protection

Department of Consumer & Employment Protection
219 St Georges Terrace
Perth WA 6000
Locked Bag 14
Cloisters Square WA 6850
Phone (08) 9282 0777
General Information & Advice 1300 304 054
Web site: www.docep.wa.gov.au

Be clear about the problem, i.e. the bike is faulty to the point of being unsafe, be clear about the efforts you've made to give theshop a chance to rectify the problem, and be clear about exactly what you want done about it whether it be a full refund or replacement.