Beneficiaries of cashed-up trusts are in the Australian Tax Office's sights, with tough scrutiny of expenses and income splitting under way ahead of a new draft ruling on reimbursement agreements.

Accounting firms and lawyers say enforcement of anti-avoidance measures under section 100A of income tax law has seen trustees of family trusts required to detail distributions to beneficiaries, including adult children, to ensure they comply with rules about "ordinary family dealings".