@Zoe31
I can see this mix up as being slightly also your responsibility in telling them they were 20 minutes away when you did not know their arrival point. I would nicely ask the guests if they preferred to stay closer to the airport on the first night and have daylight to enjoy their travel to your area. You can suggest that perhaps after a long day of travel they would feel more refreshed the following day and better able to enjoy their holiday if they had accommodations they could reach earlier on the first evening. It would be a shame to pass through x area of Tasmania and not enjoy the spectacular scenery - something like that.
I lived in Tasmania for a time and can vouch that it would be better to drive in the daylight! There used to be a pub near Cradle Mountain with a wombat named Harry who lived in a basket near the fireplace. He would roam around the pub visiting the patrons. Too cute!
Anyway, I would offer to alter the reservation and not charge them for the first night if you do not want to be disturbed with a late check in. You can put 80% of the blame on the guest for not engaging you for help with their arrival plans but some of the blame does have to fall to you as you did not clarify how long it would take to arrive and therefore provided a potential check in time within the allowed window. Suddenly the guest is left in a less than optimum position. This is mostly miscommunication and it is gracious if you can offer them a way out of the mistake that might be to the benefit of both of you. If they do not want to alter the booking, I would honour it and be more diligent in the future.
Maybe not what you wanted to hear but perhaps one nights fee is worth a good sleep?
Love Tasmania and would visit there again in a hearbeat, Lisa