I am getting 90% in 5.2 and my teacher initially said they would make sure I get into extension.
For Mathematics Advanced alone, you would need to have studied 5.3 as a prerequisite. As NESA's website says:
The Mathematics Advanced Year 11 course has been developed on the assumption that students have studied the content and achieved the outcomes of the Mathematics Years 7–10 Syllabus and in particular, the content and outcomes of all substrands of Stage 5.1 and Stage 5.2, and the following substrands of Stage 5.3:
- Algebraic techniques
- Surds and indices
- Equations
- Linear relationships
- Trigonometry and Pythagoras’ theorem
- Single variable data analysis
and at least some of the content from the following substrands of Stage 5.3:
- Non-linear relationships
- Properties of Geometrical Shapes.
For Extension 1, the prerequisite is also having studied all the above, including:
- Polynomials
- Logarithms
- Functions and Other Graphs
- Circle Geometry.
If you have not touched any of 5.3, much less the optional substrands in the list just above this sentence, then the expectation is that you will find Extension 1 (and also Advanced) difficult. Completing only 5.1 and 5.2 is the prerequisite for Mathematics Standard, which is why your mathematics faculty have said you can only do Standard, as those are the only requirements you have fulfilled.