ASX can only take action against a listed company if it is doing or failing to do something that breaches ASX's Listing Rules.
There are provisions in the Listing Rules that prevent a major shareholder from acquiring a substantial asset from, or disposing of a substantial asset to, or taking up an issue of shares by, a listed company without shareholder approval. The major shareholder and its associates are not allowed to vote on the resolution approving the transaction. If that type of conduct is occurring, ASX can and will take action to enforce the Listing Rule requirements for shareholder approval.
The directors of a listed company generally are subject to a statutory duty to act in the best interests of the company and not to favour the interests of one particular shareholder over others. However, that statutory requirement is not one that ASX can enforce.
If a listed company is established in Australia (as most ASX listed companies are), issues to do whether its directors are complying with their statutory duties under the Corporations Act are regulated by the Australian Securities and Investments Commission (ASIC), rather than by ASX. Any concerns or complaints about a suspected breach of duties by the directors of the listed company should be directed to ASIC via its complaint portal.
If a listed company is established elsewhere, issues to do with whether its directors are complying with their statutory duties will be regulated by the ASIC-equivalent regulator in the place where it is established. Any concerns or complaints about a suspected breach of duties by the directors of the listed company should be directed to that regulator.
More information about the types of matters that are regulated by ASIC under the Corporations Act and the ASIC Market Integrity Rules, and those that are regulated by ASX under ASX's Operating Rules, is available here.