Everyone guesses. The Code of Practice gives you a starting ratio, and construction counts as high risk.
The model Code of Practice suggests one first aider for every 25 workers in a high risk workplace, and one for every 50 in a low risk one. Construction is high risk.
These are a starting point, not a ceiling. The real duty is that first aid is available to every worker, on every shift, within a time that matters - which is why shifts, remoteness and a site spread over several levels all push the number up.
A first aider you cannot evidence is a first aider you did not have. The Registers Library has the first aid register, the training and competency register and the licence register - the three an inspector asks for together.
Based on the ratios in the model Code of Practice First aid in the workplace, which most Australian jurisdictions have adopted. Your regulator, your principal contractor or your contract may require more. This is a starting point for a proper first aid risk assessment, not the assessment itself, and it is not legal advice.
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The tools above are free and yours to use. When you need the paperwork behind them reviewed by someone who has run sites, Obras HSE Services is a live consulting business in south-east Queensland.