Mr Speirs said on Saturday he would step down later in October after police arrested him and raided his home.Â
Police later confirmed the 39-year-old was arrested at Berri on September 26 and charged with two counts of supplying a controlled substance between August 2 and 3 and on August 9.
"Following this his residential premises was subsequently searched," they said.
The police statement followed Mr Speirs' post on social media announced he would formally step down as the member for Black, which he has held since 2018, when parliament resumed on October 15.
"Since allegations of illicit substance use in my home became public, my home has been raided by armed police, I was arrested and I was questioned," he said in a video.
Mr Speirs said he did not believe he could "adequately and effectively serve my local community while also dealing with this matter, protecting my mental health and supporting my family and friends".
He said he would suspend his Liberal Party membership and thanked his colleagues for their support.
"For me it's time to move on, I want to rebuild myself over the coming months and into the future," Mr Speirs said.
"I want to be a better son, I want to be a better friend, to be honest, I want to be a better person.
"That takes time, and I believe it's best done away from public life."
Mr Speirs was granted bail and will face court at a later date.
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