NPHET 'hasn't done their job properly' when it comes to COVID-19 mitigation measures that aren't restrictions, according to Professor Anthony Staines.
He says NPHET has been "consistently behind" other countries when it comes to measures such as antigen testing and face masks.
However, he says a phased reopening would be the "sensible approach" given the situation we currently find ourselves in.
The DCU professor was speaking as attention turns to the easing of restrictions such as the 8pm curfew for indoor events and the hospitality sector.
There's been cautious optimism in recent days that Ireland has reached the peak of the Omicron wave, with daily case numbers gradually decreasing.
Overall hospital figures have remained high, but not at the same level seen during last January's surge in COVID-19 cases.
ICU numbers, however, have remained relatively stable since Christmas.
NPHET's due to meet this week, with Government leaders expressing optimism that some of the current restrictions can soon be eased.
Tánaiste Leo Varadkar yesterday told RTÉ yesterday that the aim is to have a quicker and more ambitious reopening than last year, but it would not happen at a "risky or reckless pace".
On Newstalk Breakfast, Professor Staines - Professor of Health Systems and Public Health at DCU - said a phased reopening is the "best strategy" in the current situation.
However, he said it's "very unfortunate" we've ended up in a situation where restrictions needed to be reimposed in the first place.
He suggested NPHET has chosen "not to do their job properly", in areas such as data gathering and mitigation measures that aren't restrictions.
He observed: “I don’t think NPHET has handled this very effectively from the start. At the beginning, everyone was very much at sea - but many other countries worked out what was going on a lot faster than our country.
“We’ve only come to antigen testing in the last few months… we still don’t have clear advice on masks and mask-wearing… and we’ve had the mantra that schools are safe until suddenly they weren’t.
“NPHET has been consistently behind the science and behind the evidence.”
Professor Staines said experts and academics here have been stuck looking at international evidence around the spread of the virus in areas such as the hospitality sector.
He said the Irish data shows there’s lots of spread in households and schools, but also “a lot of spread [where] we don’t know where it’s coming from.”
He stressed that hospital admissions are still quite high, and the number of COVID-related deaths is higher than it was a few months ago.
While he said he hopes the hospital numbers won't translate into ICU admissions, he added: “The virus causes significant illness in a lot of people, and there is increasing evidence about the biology of what they call long COVID.
"It’s a virus in a proportion of people that leads to quite a systemic disease… we don't know for how long, but it can be quite severe.”