Don’t take it up with the manufacturer initially. Under Australian Consumer Law the responsible party is the retailer to address your issues. This rusting seems to be a reasonably common complaint with these units possibly making it a design/manufacturing fault. I would if it was my oven be seeking a full refund from whomever I had purchased it from and choosing a different oven from a different manufacturer.
There is lots of advice about how to get formal in this process of obtaining your legislated rights. Phone calls as @PhilT remarks are just “idle chit chat” but may be the easiest starting point, but if they are not forthcoming in response don’t waste any more time in that manner. Put your claim in writing to the retailer (email is acceptable), there are lots of links that can be searched for here both made available by CHOICE and the ACCC as templates to produce effective correspondence. Set a reasonable time for a response eg 7 or 14 days for the retailer to respond in your favour. If they don’t you will need to consider going to your State Civil and Administrative Tribunal to secure your rights. Seeking some free legal advice is also very strongly recommended and there are some places linked on this site that do offer free support (see Free Legal Advice Centres).
