The P3a is an event-related potential (ERP) component believed to reflect an attention-switch to task-irrelevant stimuli or stimulus information. The present study concerns the automaticity of the processes underlying the auditory P3a. More specifically, we investigated whether the auditory P3a is an attention-independent component, that is, whether it can still be elicited under highly-focused selective attention to a different (visual) channel. Furthermore, we examined whether the auditory P3a can be modulated by the demands of the visual diversion task. Subjects performed a continuous visual tracking task that varied in difficulty, based on the number of objects to-be-tracked. Task-irrelevant auditory stimuli were presented at very rapid and random rates concurrently to the visual task. The auditory sequence included rare increments (+10 dB) and decrements (-20 dB) in intensity relative to the frequently-presented standard stimulus. Importantly, the auditory deviant stimuli elicited a significant P3a during the most difficult visual task, when conditions were optimised to prevent attentional slippage to the auditory channel. This finding suggests that the elicitation of the auditory P3a does not require available central capacity, and confirms the automatic nature of the processes underlying this ERP component. Moreover, the difficulty of the visual task did not modulate either the mismatch negativity (MMN) or the P3a but did have an effect on a late (350-400 ms) negativity, an ERP deflection perhaps related to a subsequent evaluation of the auditory change. Together, these results imply that the auditory P3a could reflect a strongly-automatic process, one that does not require and is not modulated by attention.