Hi Nelli,
That is disappointing on the part of the auction house- what do they think has happened to the painting?
I have a few follow up questions:
1. Can you please give some more information about how and why they had your painting without any paper work or contract signed to act as your agent in the sale?
2. How much is the painting worth? Have you had it independently valued?
Australian consumer laws generally only apply to goods that are worth less than $40,000, unless "they are of a kind ordinarily acquired for domestic, household or personal use or consumption". I think it would be difficult to show that a painting is ordinarily acquired for domestic or personal use.
If your painting is worth less than $40,000 then one of the ACL guarantees that protects you is that a business service is provided with due care and skill. If a business breaches a guarantee, then you as the consumer have certain rights, such as the right to compensation.