It is highly recommendable that you apply for an Aussie e-visitor.

Should you arrive in SYD without this visa, and then decide you wanted to go landside, I am not sure that you would be able to obtain a visa on arrival...or at the very least, your processing at immigration could take longer, and attract extra attention.

Best to play it safe. It costs nothing (assuming you have a UK/EU passport) and takes only a few minutes to fill out the form. The visa is granted almost immediately (usually) and remains valid for a year from the date of grant.