That is not true for a serious offence like this. The police will initially lay a charge on the basis of what they think the drug is, through reagent testing and obviously how it appears. Then the drugs get sent off to a lab to be tested. A certificate will come back telling them exactly what drugs were in the substance, the exact quantity and the purity. They then can amend the charge if it turns out to be something else.
Where this might not be done is for small amounts and the person pleads guilty to what the police allege they possessed. If you plead not guilty, then regardless of the amount you had they must test what was seized.
Drug laws in all states are very weight dependant (in terms of the different types of charges that can be laid and also perhaps other consequences that might follow, such as automatic confiscation of assets) and the purity of the drug, especially for powder drugs, is very relevant for sentencing. The police must have this information and the information must be exact if they are going to prosecute someone for drug offences beyond mere possesion. The only way is to have the Government Laboratory test the samples obviously.