New Zealand Law
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The best way to start may be a complaint to the Labour Inspectors through the Dept of Labour. They can investigate wages and see if they comply with the legislation, and if you are being paid correctly. They are a government service so this doesn't cost. If they are unable to help you could claim a personal grievance based on the sleepover cases, that you are being exploited. From your description, I think your employer is underpaying you.