Families SA workers effectively removed a requirement that Chloe Valentine’s teenage mother abstain from taking drugs, an inquest into the four-year-old’s death has heard.

Chloe died of massive head injuries in January 2012 after being forced to ride a motorbike that repeatedly crashed over a three-day period in her Adelaide backyard.

An inquest into the girl’s death has heard her short life was marred by horrific squalor and chronic neglect but she was never removed from her drug-using mother, Ashlee Polkinghorne.

Social worker Nicholas Ratsch told the inquest on Friday he helped Polkinghorne draft a “safety plan” in late 2007 to ensure infant Chloe received proper care when Polkinghorne was affected by drugs.

Polkinghorne, who was 16 at the time, agreed that she and her then-partner would ensure there was a sober person home with Chloe when they were drug-affected.

The plan effectively overrode a previous agreement, brokered by case workers, in which Polkinghorne pledged to cease all drug-taking.

State coroner Mark Johns questioned why Ratsch would “muddy the waters”, saying the new plan had effectively condoned Polkinghorne’s drug use.

“It doesn’t in any way start with discouragement not to use drugs, does it?,” Johns asked.

“It’s almost implicit … that they will use drugs.”

Ratsch said the agency had never condoned drug use but it was difficult to prevent because some substances were becoming decriminalised.

“It’s clear that drug-taking was not the preferred option,” he said.

But counsel assisting the coroner, Naomi Kereru, said that decriminalisation was irrelevant because Polkinghorne was not even an adult at the time.

Polkinghorne, 22, and a later partner, Benjamin McPartland, 28, were jailed this year over Chloe’s death after pleading guilty to manslaughter through criminal neglect.

The inquest continues.