As said the $Aus hovers around 25 rupees. You can change cash at the airport - all the major banks have tellers as you come out. Ask for some small notes as you'll find 100, 200 and even 500 rupee notes better in most instances.
They stamp your passport with the visa when you enter at immigration.
Some current prices which you might find of interest :
Phoenix Beer approx 39mr for 700ml bottle
Bottled water from about 12mr per 1.5 litre bottle at supermarkets.
Rum about 100mr upwards. You can get Goodwill for about 90mr which is popular with locals. Try a few different ones for yourself.
Bread - baguette around 11mr
Cigarettes - about 90mr a pack (very cheap compared to Australia)
Petrol - was 39mr a litre last month - may have gone up.
Restaurant meals from around 120mr upwards. You can still get decent Bryani for about 85mr and mine (mein) often cheaper. Dining out is relatively cheap compared to Australia.
Car hire ranges from about 700mr / day up, depending on season, etc. You may be able to talk it down in August as it's not peak season.
Bus from Grand Bay to Port Louis about 30mr each way per adult.
Some products like decent meat, some fish etc is similar price to Australia and a lot is much cheaper - sugar, eggs, bread, butter is much cheaper (whereas margarine is about same price).
You'll enjoy your 3 weeks I'm sure.