Is it possible to contract out of the proportionate liability legislation?

It depends on the contract and the jurisdiction.
Tasmania, New South Wales and Western Australia include a contracting out provision.
There is no express provision, either way, as to contracting out in the proportionate liability legislation introduced in the Australian Capital Territory, the Northern Territory, South Australia or Victoria, nor in respect of the Commonwealth provisions contained in the Trade Practices Act 1974.
Contracting out is prohibited in Queensland, see the Civil Liability Act 2003 (Qld), s7(3).
In most cases where proportionate liability legislation applies, in order for a claimant to recover 100 per cent of the claimant's loss, the claimant must sue each and every wrongdoer who contributed to that loss.
No wrongdoer is liable to the claimant for more than that wrongdoer's share of responsibility for the loss.
The following clause was sufficient to contract out. Aquagenics Pty Ltd v Break O'Day Council¹⁾
- Approval to subcontract shall not relieve the Contractor from any liability or obligation under the Contract.
- Except where the Contract otherwise provides, the Contractor shall be liable to the Principal for the acts and omissions of subcontractors and employees and agents of subcontractors as if they were acts or omissions of the Contractor.
In Curtin University of Technology v Woods Bagot Pty Ltd ²⁾ the Court found that the legislation did not apply to arbitration unless they contractually agreed to do so.

Proportionate Liability Legislative Provisions of Interest for Arbitration

¹⁾ , ⁴⁾ [2010] TASFC 3

²⁾ [2012] WASC 449

³⁾ [2013] HCA 10

⁵⁾ [2009] VSCA 245