It's not just a matter of your listing being self-contained. What the govt. is trying to do is discourage people travelling, as when they do they can spread the virus around on their travels. And although you don't need to have any direct contact with your guests, they will probably be going out and about in your community, stopping along the way- do you want to take the chance and responsibility of them bringing the virus there? This is exactly what the Public Health Order is designed to combat. If people ignore it, then you ca look forward to even more stringent orders in the near future.
If it were me, I'd get them to cancel and refund them, because I believe in being part of the solution regarding this virus, rather than doing things that could encourage more infection. Just because it's worded "should" doesn't mean it's something to be ignored. Something called a "order" seems like a pretty clear mandate that you and the guests should follow.
@Nikki339