The AMWU worker likely used alcoholic mouthwash. But it’s not a ‘false positive’, and it raises questions

On Thursday morning (9 April 2026) The Australian reported a story that had been leaked to them about an AMWU (Australian Manufacturing Workers' Union) official who had been denied entry at a BHP site for failing two alcohol tests.

A statement was given by AMWU Secretary Steve McCartney. Mr McCartney stated:

“The mouthwash caused a likely false-positive reading of 0.04 BAC (blood alcohol concentration) on breath-testing equipment at the site’s gatehouse. A second test was conducted 10 minutes later which recorded a 0.02 BAC reading… The human body cannot metabolise alcohol quickly enough in 10 minutes to explain the difference between these two results.”

I have two corrections.

1. It is not a false positive. If a person introduces ethanol into their mouth and ethanol is detected in a breath sample, that is simply a positive result. A false positive would imply the instrument malfunctioned or was not calibrated. There is no evidence of that here.

2. At no point was the worker’s BAC (blood alcohol concentration) measured. What was measured was BrAC (breath alcohol concentration). They are related concepts, but they are not interchangeable, and using them interchangeably muddies the discussion.

Mr McCartney is correct on one point. The human body eliminates alcohol at a rate of roughly 0.003 g/210L over 10 minutes. On that basis, a drop from 0.040 to approximately 0.037 might be expected in someone with a genuine systemic alcohol concentration. A drop to 0.020 is not physiologically consistent with elimination.

But that leads to the more important question: what is it consistent with?

A rapid decline of that magnitude over a short interval is entirely consistent with transient oral alcohol contamination. In simple terms, alcohol present in the mouth but not yet absorbed into the bloodstream. Breath testing devices are designed to sample deep lung air, but if alcohol is present in the oral cavity, it can contaminate the sample and produce an elevated reading that falls quickly as the alcohol dissipates.

That is not a fault of the instrument. It is a limitation of the sampling process, which is why many testing protocols include an observation period to ensure the mouth is clear of recent contaminants. Had that protocol of been observed, I have not doubt the AMWU would then be up in arms about 'delaying persons coming onsite'.

So the science here is straightforward:

• The initial reading of 0.040 reflects ethanol present in the breath sample

• The rapid drop to 0.020 is not consistent with metabolism

• The pattern is consistent with alcohol in the mouth rather than the bloodstream

None of that is controversial.

Which brings us back to the part that actually matters.

This is allegedly a seasoned union official. Someone who would be familiar with:

• Zero-tolerance alcohol policies on Tier 1 mining sites

• The inevitability of drug and alcohol testing at entry

• The basic expectation that you do not introduce anything into your mouth immediately prior to testing

So why was he doing it?

If the explanation is accepted at face value, then what is being asserted is that an experienced individual:

• Used an alcohol-based mouthwash

• Immediately prior to attending a site where testing is guaranteed

• Without allowing even a minimal clearance period

That is not a scientific issue. That is a judgment issue.

Because regardless of whether the ethanol came from whisky or mouthwash, the outcome is the same and the AMWU worker has failed to comply with the policy of the PCBU (BHP). This means the PCBU (BHP) has a legal duty to ensure he does not come on site as he may pose a risk to himself and others.

BHP’s position is equally straightforward. They operate under a zero-tolerance framework. That means:

• No interpretation

• No benefit of the doubt

• No distinction between sources at the gate

If you cannot pass the test, you do not enter the site.

That policy exists for a reason. These are high-risk environments where even small impairments can have serious consequences. The system is designed to be blunt because the stakes are high.

So while the union is attempting to frame this as a “false positive”, the science does not support that characterisation. The device detected ethanol. The numbers behaved exactly as ethanol in the mouth behaves.

The only real question left is not whether the reading was wrong.

It’s why someone who should know better put themselves in a position where that reading was inevitable.