Based on cognitive load theory and the "transient information effect," this paper investigated the "modality effect" while interpreting a contour map. The length and complexity of auditory and visual text instructions were manipulated. Experiment 1 indicated that longer audio text information within a presentation was inferior to the equivalent longer visual text information demonstrating a reversal of the modality effect due to transient information imposing a heavy working memory load. However, the expected modality effect was not obtained from the equivalent shorter auditory text presentation compared to shorter visual text information. It was hypothesised that the shorter text still contained too much auditory information for working memory to readily process. Experiment 2 further decreased the shorter auditory text information which then resulted in a traditional modality effect including a modality by text length interaction in which shorter, audio-visual information was better than visual only information but longer, audio-visual information was worse than visual only information.