Opposition Leader Bill Shorten has defended penalty rates, saying the extra income is the difference between parents sending their children to private schools over public schools.

Mr Shorten on Monday said millions of Australians would be enjoying the Labour Day public holiday, which is in place in several states across the country including New South Wales and the ACT.

On a public holiday, a casually employed cafe worker aged 21 or more serving coffee earns $52.23 an hour, according to the Fair Work Commission's online pay calculator. Small businesses say the high wages force many cafes and small stores to close, because opening would mean operating at a loss.