Most mainstream lenders require your ABN to have been actively registered for at least 2 years before they'll consider a full-doc self-employed application. This isn't a regulation — it's a lending policy adopted by most banks and mortgage insurers.
It's very difficult through mainstream lenders. Specialist non-bank lenders may offer low-doc products but expect 30%+ deposit and higher rates.
No. PAYG contractors who receive payslips with tax deducted are not assessed as self-employed, so the ABN rule doesn't apply.
It's a lender policy, not a legal requirement. No regulation mandates 2 years of ABN. However, it's so widely adopted that it's effectively an industry standard.