How can a Thai national with dual citizenship be fined for overstaying using an Australian passport?

A Thai national holding dual citizenship may face fines for overstaying if they entered Thailand using their Australian passport instead of their Thai passport. When entering on a foreign passport, they are treated as a foreign national by Thai immigration, which does not acknowledge their Thai citizenship for immigration purposes. Hence, failure to inform immigration about their dual nationality can result in penalties for overstaying. It's noted that they could have applied for a year-long extension based on their Thai citizenship for a fee if they had used their Thai identity.