Not if you're under 21. The relevant rule in Article 9 - Change of Association in the FIFA eligibility rules is:
"2. A request to change association may be granted only in the following circumstances:
...
c) the player:
i) was fielded in a match in an official competition at “A”international level in any kind of football for his current association;
ii) at the time of being fielded for his first match in an official competition (at any level) in any kind of football for his current association, he held the nationality of the association which he wishes to represent;
iii) at the time of being fielded for his last match in an official competition in any kind of football for his current association, he had not turned 21 years old;
iv) was fielded in no more than three matches at “A” international level in any kind of football for his current association, whether in an official competition or non-official competition:
v) at least three years have passed since being fielded for his last match at “A” international level in any kind of football for his current association, whether in an official competition or nonofficial competition; and
vi) has never participated in any kind of football at “A”international level in the final tournament of the FIFA World Cup or a final tournament of a confederation competition."
Robertson doesn't fulfil condition 2.c.i) because he hasn't been fielded in an official competition at "A" international level for Australia. Accordingly, the three year requirement before switching does not apply to him. Hypothetically, it would apply to him if he had played an Asian cup qualifier when he was 20 but had played no other games.
