What Pa level should I use...
That really depends what you're trying to do with it.
Is this data just missing near the surface?
The pressure level 1
hur data in your ACCESS model plot are missing because the surface pressure in those grid boxes is less than 962.5 hPa lower bound of that layer. In other words, that atmospheric layer doesn't exist above those grid boxes, so properties of it like temperature and humidity are not defined. If you look at pressure levels 2, 3, etc, you should notice less and less of the area is undefined.
To be honest, looking at pressure level 1 isn't very useful for this reason. In some places it'll be undefined, in some places it'll be inside the boundary layer and in some places it'll be outside, making interpretation difficult.
If you're looking at a single level this close to the surface, then you probably really want to look at the screen-level humidity variable,
hurs. That will be defined everywhere, is more comparable between models and is usually more relevant when looking at surface level impacts and applications. The same is true for temperature variables