Drug and Alcohol Testing is not a perfect science, it is also not immune from corruption or cowboy operators - largely due to a lack of regulation.
When a drug test is performed in the field, there is no legal way to say you are 'positive for a drug' (other than alcohol) so if you are fired on the spot for a not-negative result I would consider that to be unfair dismissal as no validation by a NATA accredited laboratory could have occurred.
Whilst I am very careful and certain beyond reasonable doubt before I say a person is positive for a drug, not all testing officers are and may provide misleading information to your employer.
If you have been accused of drug use by police, your employer or a regulatory body (i.e. AHPRA) and believe the test is a mistake, you can request the NATA accredited body (i.e. Sullivan Nicolaides) to retest the specimen or otherwise transfer it to another NATA accredited laboratory to be tested in accordance with Australian Standards.
If you believe your sample may have been tampered with, you can request a field and laboratory test from any drug testing body of your choice, at your own expense. This must be done as soon as practicable if you intend to use it as a defence.
NRDAT have identified AHPRA's testing processes to be non-compliant with Australian Standards and advised them of this in early 2021. They have undertaken to review their processes. It would be possible to appeal any failed drug testing test and relevant disqualifications of Health Provider registration that occurred between 2020 and 2021.
For any dismissal from a public agency, applying through your states Tribunal is possible. For dismissal from a private employer, you can seek redress from the employer directly and/or the Fair Work Commission.