This article discusses the risks for parties to international mergers of ignoring or underestimating voluntary merger control regimes. It highlights key points for business to keep in mind, including that voluntary notification regimes are best viewed as having quasi-mandatory notification requirements, the ability of authorities in such regimes to commence unilateral reviews, the potential for reviews in voluntary regimes to disrupt or hinder global clearances, and the enhanced risks arising from global authorities being increasingly interconnected. The article discusses these risks in the context of the Australian Competition and Consumer Commission’s (ACCC) review of Google’s acquisition of Fitbit, and also highlights potentially significant reforms proposed for the Australian merger control regime.