Short answer. It can be, but the protection is not automatic. Article 354 of the Revised Penal Code presumes every defamatory imputation malicious, then excepts a private communication made in the performance of a legal, moral or social duty. A complaint filed properly with the right office normally falls within that exception.
What the law says
Every defamatory imputation is presumed to be malicious, even if it be true, if no good intention and justifiable motive for making it is shown
Revised Penal Code, Article 354 — Requirement For Publicity. Read the full provision →
What the law says
A private communication made by any person to another in the performance of any legal, moral or social duty
Revised Penal Code, Article 354 — Requirement For Publicity. Read the full provision →
Start from the presumption, not from the truth
The article begins from an uncomfortable place: a defamatory imputation is presumed malicious even if it be true, unless a good intention and justifiable motive is shown. Many people assume that proving their accusation was accurate ends the matter. Under this article it does not, on its own. Truth is powerful and often decisive in practice, but the structure of the provision is that malice is assumed first and then rebutted. What rebuts it is either falling within one of the two listed exceptions, or showing that you spoke with good intention and a justifiable motive. That is the frame every libel defence in this situation is built on.
The privileged-communication exception
The first exception covers a private communication made by one person to another in the performance of any legal, moral or social duty. A complaint against a public official, addressed to the office that has authority to investigate or discipline him, is the standard example of a communication made under a duty. Three things typically need to hold. The complaint must go to a person or office with a genuine interest or duty to act on it. It must be confined to the matter complained of, rather than wandering into insults about the official's private life. And it must be made in good faith, on a real belief that the facts are as you state them.
Where the protection is usually lost
The exception protects the complaint; it does not protect a campaign. Sending copies to the official's neighbours, posting the accusation on social media, or handing it to reporters while the case is pending takes the statement outside a private communication and puts you back inside the presumption. The same happens when the language goes beyond what the grievance requires — calling the official a thief in general terms rather than describing the specific act. Reckless indifference to whether the accusation is true also defeats good faith. Note as well that publishing defamatory material online can attract liability under the cybercrime law, which is a separate statute with heavier consequences.
The second exception, and what to do
The article separately protects a fair and true report, made in good faith, without any comments or remarks, of judicial, legislative or other official proceedings that are not confidential, and of acts performed by public officers in the exercise of their functions. That covers repeating what happened at a hearing accurately, but the words without any comments or remarks are strict. Adding your own conclusions can forfeit it. If you are drafting a complaint, keep it factual, keep it addressed to the proper office, keep your supporting documents, and resist the urge to circulate it. If you have already been charged, no outcome can be promised — bring the complaint and the papers you relied on to counsel.