I got one on sale for about $15 from Bunnings a bit over a year ago. Works perfectly well although there are a couple of little gotchas. The pump handle can come off quite easily which can be a nuisance when you're halfway through spraying. Probably easy(ish) to fix with a drill and split pin. The straps are a bit thin and can become uncomfortable and it can be a bit of a nuisance to put on. Otherwise it works fine and dandy.

I rekon you can't go past Solo. I've got one and Dad always used one. They are great.

https://www.solonz.co.nz/products/catalogue/backpack-sprayers

From my experience, avoid the cheapest of the backpack sprayers - the bend of the pump handle introduces a weak point that breaks after a year or so usage.

The Solo sprayers don't have this problem, and feel a lot better made. 

I bought the Solo Backpack Sprayer with Padded Straps 15L from the Warehouse it should suffice.  Thanks for your help.