Hello Max,
I'll try to provide some guidance. You can review the DIMIA fact sheet on the Character Test here:
http://www.immi.gov.au/facts/79character.htm
You didn’t provide details of your 1983 record but it seems minor and as it was so very long ago it is extremely unlikely to be an issue if there has been no problems since (bar the caution). However, it is unclear whether or not you have lodged your visa application as yet. If you haven’t, you will note you are required to declare on your application whether you have EVER been convicted of a crime or offence - no matter how minor - including any now removed from official records (eg police clearance certificates). Consequently, you must declare the 1985 offence and should include an explanation that it was such a very long time ago when you were young - 20 years - that you had unfortunately overlooked mentioning this on your license application last year and as a result received a caution in 2004.
If you choose not to declare this information you could be at risk of failing the character test under ‘past and present general conduct’ due to dishonesty, if it should come to DIMIA’s attention. Therefore, not only are you required to declare this information, but it is in your best interests to do so, whether or not details show up on the police certificate.
If you have already lodged your visa application, you are obliged to notify DIMIA in writing to correct any wrong information that has been given as soon as possible after you become aware of it (use Form 1023 available here:
http://www.immi.gov.au/allforms/forms_num3.htm). (Indeed, the obligation to DIMIA to correct wrong information continues even after the visa has been granted.) Again, provide an explanation of what happened and that you have taken the required steps to correct the wrong information as you are aware you have an obligation to do so.
Please get this out in the open and stop making yourself sick with worry - or the next problem will be health!
I’ve just come across this forum and will stop by when I can.
Kindest regards,
Greta