They have made the call based on the current trends.
It's estimated the reproduction rate has risen to between 1.5 and 1.6 while the estimated 14 day incidence has increased to 122 per 100 thousand.
Professor Philip Nolan, chair of NPHET's modelling advisory group, says the country has "clearly" entered a third wave:
"If we're at a growth rate of 5% per day, and I think it's higher than that, we'll see 900 cases a day by the 30th of December and 1,300 by the 6th of January in a simple exponential projection. If our growth rate at 7% per day would be at 1,100 cases a day by the 13th of December, and 1,800 by the 6th of January."