Dozens of owners and tenants at the Opal Tower have met to explore their legal options, as a real estate expert has warned of a 50 per cent drop in the value of apartments in the western Sydney block.

On Thursday, about 100 people attended a meeting with representatives from NSW Fair Trading to discuss their rights, with angry owners and tenants breaking into separate groups to explore collective legal action.

Hundreds of residents remain displaced after the building was evacuated for the second time last week, with no update on when they can return. Photos published by the Herald on Thursday also show the extent of remediation work being done.

At the meeting, owners were discussing launching a class action against the builder and developer, while tenants discussed their options, including breaking their rental contracts.