I'd look at the latest restraints testing results - which are here:http://www.crep.com.au/crep-results.php
At 20 months you could put her into a type B/E which is a convertible booster seat (does from 8-18 kilos with inbuilt 5 point harness, then 18-26 kilos with seatbelt (H harness's are not recommended for most kids).
However, the one thing you need to watch with the convertible boosters seats is they tend to recline less and be less supportive of their heads if they are still tending to fall asleep in the car.
The highest rating seats for her age are the Babylove ezycombo (a convertible) and the Meridian tilt and adjust (a 0-18kg seat).
Then there are quite a number of 3 star seats as well.
FWIW - we have both boys in a hipod senator because they are 3 star rated AND fit well in our car. We have it reclined for ANgus (21 months) and he is quite comfy - however, in my old smaller car there wasn't enough room to recline it so we didn't use it in that car.
Things to consider are the width of the seats - we wanted to fit an adult in the same row as two car seats so preferred narrower seats (the Babylove ezycombo is really wide). We ended up choosing the hipod senator based on price vs the maxirider (both got same rating) and width.
ETA - I've heard that the convertible boosters aren't as easy to use in booster mode so I wasn't so bothered that the Senator scored less in that mode as I suspect by the time Will needs it in booster mode we will get him a dedicated booster and put the Senator in my parents car etc. If you want to use it in booster mode though - make sure you check the rating for 6 month - 4 years and for the next category up.