WA Police say meth use is down in the state, claiming drug busts and the collapse of the mining boom leaving people with less cash for illicit substances could be the cause.

Last year, analysis found WA was using about $2 billion worth of methamphetamine each year, while a national report labelled WA the highest ice-using state per capita in the nation.

But how reliable is this method of monitoring?

According to UWA's expert professor Anas Ghadouani, with today's technology you absolutely can detect the presence of drugs and their by-products in the waste system.

But when it comes to interpreting the results, and their statistical significance, things become murkier.

So how does it work?

Police say samples are collected across the Perth metropolitan area, Bunbury and Kalgoorlie over a seven-day period, once every two months.

They used to be collected in Broome and Geraldton, but that has stopped as they are no longer considered "problem areas" by WA Police.

They use experts at the University of South Australia to analyse the results, to work out how many doses are being taken per head of population.

The average dose is defined as 68.5 per cent per purity for a 100 milligram hit of the drug, known on the street as a 'point' of meth, which costs $100.

According to Acting Assistant Commissioner Pryce Scanlan, the force uses the data to shape their operational activity.

Has meth use really gone down?

According to WA Police, the state's meth use fell 26.6 per cent in Perth and 41.5 per cent in Bunbury from September to April this year.

But such a dramatic fall may not be as significant as it first appears.

Professor Ghadouani said based on the data released by police, it was hard to ascertain any trend.

"I think based on the data I have seen it's very hard to make that determination of a trend going downwards or upwards or staying stable," he said.

"I think it's important to understand these things are complex, it depends on the catchment, on the number of samples, there's lots of variations that are going on in there.

"Drawing conclusions based on data that is collected (sporadically) ... and in various places is very, very hard.

"I have been looking at some of those trends and I can tell you just by looking at it by eye, it wouldn't be statistically significant."

Total Consumption (doses/week/1,000ppl) Bunbury Kalgoorlie Broome Geraldton Perth Metro Jul-15 138.7 Sep-15 124.1 Nov-15 272.7 162.1 Jan-16 279.9 148.3 186.4 Mar-16 236.2 128.8 147.0 May-16 197.3 143.0 145.3 Aug-16 237.1 178.2 Sep-16 306.9 209.3 Nov-16 224.2 233.2 28.4 162.3 Feb-17 163.4 191.7 46.6 124.1 Apr-17 144.1 168.3 34.0 110.6

Source: WA Police

Is the data reliable?

While WA Police have not released their methodology, there are some obvious factors that need to be controlled for.

Researchers working on a similar national study for the Australian Criminal Intelligence Commission (ACIC) last year considered issues such as population fluctuation, how people metabolised the drug differently, pipe corrosion and tidal flow.

There is also a risk drugs dumped into the sewerage system during police raids are showing up in results.

However a WA Police spokesperson said this would have been obvious in daily data, over the seven-day sample period, if drugs were tossed down the toilet, as they would have appeared as a major spike.

So how bad is it really in WA?

ACIC's report looking at meth use over a one-week period in August 2016, the latest data publicly available from this body, indicated methamphetamine consumption in both capital city and regional areas in WA was higher than all other jurisdictions and far exceeded the national average.

The report also said nationally methamphetamine rates were increasing and were at historic highs.

Acting Assistant Commissioner Scanlan said he was not prepared to speculate whether WA remained the worst place for meth use in the country.

"What I am prepared to say is that we are heading in the right direction," he said.

"It's pleasing to see the amount of drug over the last three testing periods reduced."

The results from WA Police indicate in WA it has gone down, but without being able to compare them to stats from other states, we don't know if WA still retains its crown as the meth capital of Australia.

Professor Ghadouani said even then, it was hard to compare different areas, as there were so many different variables state to state.

ACIC is due to release its next report in July.