Surplus labour creates low wages and zero hours contracts.

I think is intended to be a version of "the law of supply and demand". If the supply exceeds the demand, the price falls. If the demand exceeds supply the price rises.

But that is only an approximation to the rule. You need to add in the clause "until some constraint is reached." You can't increase production of widgets indefinitely: you run out of factory space or skilled staff or raw materials. Even if you can't satisfy the demand. And the same is true in the case of labour: things like the minimum wage act as constraints. As can, or could, laws about zero hour contracts.

It is worth remembering that the minimum wage is not about the government being nice. It is about reducing the amount the government would otherwise have to spend on benefits by making the employer shoulder a reasonable part of the burden, rather than take more profit and pass the costs onto the country as a whole.