The ACT government spent almost $4 million over four years on 56 new Philips brand cardiac monitor-defibrillators, described at the time by Emergency Services Minister Simon Corbell as the "most essential piece of life-saving equipment carried on board all of our ambulance vehicles".

But, despite a period of field testing, the devices began malfunctioning on Canberra's streets, experiencing a series of failures, including battery problems, data transmission issues, and inaccurate blood pressure readings.

Those problems were reported by paramedics and the ACT government publicly assured the community in February the issues had been fixed, with the help of the manufacturer.

Yet, as recently as late last month, paramedics have again experienced problems using the new Philips equipment.

In a recent cardiac arrest case, the ambulance crew were left unaware that a defibrillator shock had not been delivered to a patient.

The shock itself was not meant to be delivered, due to thoracic impedance, but it was of concern that paramedics were left unaware.

A message alerting the failed shock briefly flashes up on the cardiac monitor screen, but it was missed.

Further changes have since been made to ensure ambulance crews are better alerted when a shock is not given.

In March, one device failed to power up due to a battery failure. The manufacturer issued a recall for the batteries in February and they are in the process of being replaced in ACT ambulances.

A month earlier, one of the machines had trouble taking a blood pressure reading, in an issue related to the cuffs of the monitor and their placement. That problem is not unique to the new Philips machines.

A spokesman for Mr Corbell said these problems have not had any adverse impact on patients, but that the ACT Ambulance Service has now written to the distributor, "clearly articulating" its expectations for the equipment's performance.

There are backups and manual techniques that allow paramedics to work around any problems.

But the lingering concerns around the equipment have raised questions about the government's handling of the purchase and their recent assurances that problems were resolved.

Shadow Emergency Services Minister Brendan Smyth said there had been worrying signs with the equipment, including product recalls, before the ACT government made the $3.8 million purchase.

He said the issues were not only of concern to the community, but also to paramedics, who he said were under pressure because of a bad purchase.

"The question is how did the process let our ambulance officers and our community down so badly?" he said.

"You would have thought the government had done the due diligence ... or found out for themselves that these machines were going to work."

Mr Corbell's spokesman said there was no current plan to replace the equipment and that continuing issues, including with the battery, were being worked on in a bi-monthly working group with the manufacturer, distributor and other users.

He said none of the issues had been linked with adverse patient outcomes.

"It is however recognised that a delay in delivering defibrillation to a patient does present a significant risk," he said.

"It is for this reason that, pending completion of the monitor battery replacement program, all front line emergency ambulance vehicles are fitted with backup defibrillation capacity."

The Transport Workers Union, which represents ambulance officers, said there had been no danger to patient safety.

Official Ben Sweaney said the ACT Ambulance Service had been transparent with members over the problems.

“We want to reassure the community that at no time has patient safety been compromised and Canberrans continue to receive the very best pre-hospital care of any jurisdiction in Australia," he said.

But Mr Smyth said the government needed to make sure the equipment worked 100 per cent of the time.

"Why has the minister allowed these machines to be maintained in operation when they are not reliable?"