A GP can provide a referral under the government Better Access to Mental Health Care initiative that will allow you to claim a rebate from Medicare for up to ten sessions in any one calendar year.
Some private health funds require a GP referral.
If you want a third party, such as TAC, Workcover or your employer to pay for your session(s), you need to obtain permission from the relevant third party before the appointment or you are asked to pay for the appointment and negotiate your reimbursement by the relevant party.
You can self refer without a referral from a GP but you will not be eligible for a rebate.