A. Estimates should be reassessed to reflect the conditions under IFRS and any new information available. This statement is in line with the principles of transitioning to IFRS, as it emphasizes the need to align accounting estimates with the requirements of IFRS and to consider any new information that has emerged since the entity last prepared its financial statements.

C. Deferred tax should be restated, based on conditions as they were perceived at the time of the earlier financial statements. When adopting IFRS for the first time, an entity should restate its deferred tax liabilities and assets based on the understanding of tax conditions and rates at the time those earlier statements were prepared.

D. Retrospective application is not optional; according to IFRS 1, a company must apply IFRS retrospectively in its entirety unless it is impracticable to do so. Retrospective application is generally required for comparability and consistency in financial reporting.

Therefore, the correct statements regarding first-time adoption of IFRS are A and C: estimates should be reassessed to align with IFRS requirements and any new information available, and deferred tax should be restated based on the conditions prevailing at the time of the earlier financial statements. Retrospective application is mandatory unless impracticable.