Before using the hose, remove the vents and spray inside with good bug spray. wait 10 minutes and pull out all the leaves and sticks that you can using your hand.

Then on a warm day (you are in summer like australia), get the hose in there. The water box is a very common place for rust. It is diabolical to fix. It will rust through to inside causing wet carpet, and it will rust through to the engine bay, which you commonly see. too. IT is known to weaken the chassis rail causing the front end to completely rip off on a sideways crash into a pole. I don't expect that most us need to worry about that though

Use plenty of soapy water too, and rinse afterwards. Then get a paintbrush or toothbrush and some rust convertor and treat all the seams. Leave it for 60-90 minutes in the shade.

Nissan never planned for these cars to last longer than 10 years, and the waterbox is evidence of that. It's not painted inside along the seams, and constructed in a way to promote killer rust in structural areas.

After the rust neutralising stuff has done its job (read the instructions) move it back out into the Sun and rinse again with water. Leave it to dry. You can finish it off with some odourless fish oil spray on the seams inside the waterbox too if you want.

The Fish oil might make your ventilation smell a little, for a week or two.