Hi Gwen
As we know, the public holidays in Qld were Friday 3 April (Good Friday), Saturday 4 April (Easter Saturday) and Monday 6 April (Easter Monday) (as confirmed on
this government webpage which lists the public holidays for each state). As you point out, the three nights you stayed covered only two of these public holidays.
If the business intended for "Peak season surcharge: $40 per night - QLD public holidays, 2-26 Jan 2015" (which is as you've extracted it) to mean days in and around public holidays during this period such as the Sunday for which you've been charged, then my first thought is that they haven't done a great job of making it clear. Do they mean the surcharge applies to Qld public holidays during the dates listed (which is how I initially read it), or is it Qld public holidays and the dates 2-26? It might be clear in their minds, but it's actually a bit confusing, to my mind anyway.
The Australian Competition and Consumer Commission (ACCC) is pretty strict when it comes to correct and accurate price display by businesses. The key point is that businesses need to err on the side of caution (you can find more detailed but accessible information from
this ACCC webpage)
One option to consider is calling them to let them know the $40 charge for the Sunday came as a surprise to you and to request a refund. If need be in that conversation you can remind them it's their obligation to make their prices clear to consumers because as the ACCC requires their prices must be ''clear, accurate and not misleading to consumers''
Let's see what the other contributors think about this too.