The accuracy of drug testing machines is under a cloud after secret documents uncovered police concerns about invalid test results.

Emails obtained under freedom of information laws between WA Police and their Eastern States counterparts reveal less than six months after the Draeger Drug Test 5000 machines were rolled out across WA in 2014, police reported “quite a few issues”.

They also show almost one in three saliva testing kits were failing to return adequate readings.

"The majority of these are machine faults and invalid tests. We have also found that about 30% of our STKs (saliva testing kits) are not working sufficiently - the adequacy indicator fails to turn blue even though there is plenty of saliva there," the email from a WA Police officer, dated November 24, 2014 reads. “We have played around with different batches and also the height of the gap, but nothing seems to help.

“Unfortunately, our legislation is very particular, so any equipment or consumable faults present a huge roadblock for us in terms of charging the subjects.”

A WA Police spokeswoman confirmed there was no legislation to allow the analysis of the initial road side test by the chemistry centre for an invalid or negative result from the Draeger.

“The machines are covered by Warranty and supported by Draeger. When issues arise they are repaired and reserve or replacement machines are used to sustain drug testing operations,” she said.

"WA Police continue to examine new and existing technology and have recently instigated expressions of interest to industry to exam current opportunities”, the spokeswoman said.

In September 2014, a NSW Police report found the machines were failing to pick up on positive readings for THC despite a positive initial roadside swab test and an admission from the driver they had used cannabis in the past 24 hours.

“This is a substantial rise from the negative detection rate with the superseded COZART instrument," the report said.

“Other police jurisdictions report similar pattern of negative results in respect to THC on the Draeger instrument.”

Queensland Police reported a similar issue about the same time and reduced the detection rate in the machine from 25ng per one millilitres of saliva to 5ng.

NSW followed suit and also adjusted their detection rate.

Despite the tweaking, in June last year, NSW Police acknowledged the machines were not “picking up THC 100% of the time”.

“All States that use this instrument have having similar difficulties. It is not unique to NSW,” an email obtained under FOI reads.

NSW Police Assistant Commissioner John Hartley told Seven News that officers there were experiencing an accuracy rate of approximately 67 per cent with their Draeger machines.

“It’s not as good as we’d like, we’re working with the manufacturers to improve that rate, we’re not sure exactly what’s causing the problem,” Mr Hartley said.

The purchase of 20 Draeger machines by WA Police in 2014 saw 10,903 motorists undergoing a preliminary drug test, with 1128 delivering positive results.

In 2015 WA Police conducted 27,515 drug driving tests. Ten per cent of those came back positive to drugs such as methamphetamine, cannabis and MDMA.

Late last month Police Commissioner Karl O’Callaghan said he was not surprised at the high number of motorists registering drugs in their system.

“I think I’m concerned but not necessarily surprised, because we do know, particularly the main one that comes up these days is methamphetamine, then THC or cannabis after that," he said.

“It’s not a surprise, but it is a concern, definitely.”