Do I need to investigate this?
Does the alleged conduct justify disciplinary action (including dismissal)?
Before beginning workplace investigations, consider whether or not if the conduct alleged is verified, you intend to discipline/counsel/take some sort of action against the employee. If the allegations were proven but you would not take prompt any action against the employee, then you may not need to investigate.
Do you have an obligation to investigate the allegations under an enterprise agreement or your workplace policies?
Review the relevant workplace instruments (organisation policies / enterprise agreement / awards). In many circumstances these instruments will specify when an investigation should be conducted and the manner in which it should be carried out. In particular, look out for ‘Dispute Resolution’ clauses/policies.
Could this matter lead to serious legal repercussions?
If it is alleged that a workplace law or workplace instrument has been breached then you may need to investigate.
Is the matter minor?
If the complaint is minor, then it may be best dealt with at a HR level. Check workplace instruments to see whether there is discretion to consider the seriousness of a workplace complaint when deciding to investigate.