The region's three metro district health boards issued the alert on Wednesday, saying the turnaround time for Covid tests was currently longer than normal because of "sustained elevated demand".
"Please advise patients it can take up to five days to receive a result," the notice read.
Relaying this information would reduce the volume of inquiries to labs, Healthline, Community Testing Centres and primary care from people following up their results.
Doctors were told work was underway to reduce turnaround times, with revised testing guidance from the Ministry of Health and increasing laboratory capacity.
National Haurora Coalition clinical director Dr Rawiri Jansen said the situation was deeply concerning and a signal the testing system was beginning to break down.
"In my opinion it's deeply concerning. It's a problem if processing test delays mean that people who are positive don't know that they are positive for several days."
He said the delay risked infected people becoming very unwell while waiting for the result or unwittingly spreading the virus. Testing delays would also see illness levels reflecting the numbers from several days ago.
"If we're receiving the current numbers we're receiving it on the basis that is contemporaneous, it's timely, and if it's not then we can get into trouble quite quickly," he said.
"There will be a crescendo of these delays and many more people getting sick."
He warned in two week's time there would be 100,000 people trying to get tested to cross Auckland's border, travelling in and out of the country's Covid hotzone.
From December 15, people who are not vaccinated will be required to have a negative test 72 hours before leaving Auckland to visit other parts of the country. This requirement will remain in place during the core summer period until January 17.
"Potentially our system is going to have a huge burden on it in seven days. If we have people who are spreading it who don't know they are positive it will come back to us in an uncomfortable way, landing at a time when people are getting prepared to cross borders, Jansen said.
"If we're still in a place we are catching up on tests and having test delays I think we need to call that out early.
"It seems to me that this is an early sign of a potentially big challenge."
Jansen said it was important to implement a new strategy in our testing settings so those who were symptomatic, close contacts, or at risk of being positive, could be prioritised in the circumstances.
The best way to counter the potential breakdown would be to keep alert levels in place.
"Politically it's not going to run but it is the appropriate public health response if our testing capacity has been breached.
"If we've got any sense of a crescendo of cases then the actual correct response is have a circuit breaker."
A Northern Region Health Coordination Centre (NRHCC) spokeswoman said the recent increase in surveillance testing, including for teachers and permitted workers crossing the boundary, meant the laboratory network in the region had been under more pressure, and the volume of tests received had exceeded local capacity consistently since early October.
"To support the increased demand, samples continue to be sent for processing at labs around the country," she said.
Over the past seven days, labs had processed a rolling average of more than 15,150 tests per day. This was compared to about 9000 per day eight weeks ago.
The spokeswoman said although most samples were resulted within two days, a small number were taking longer, and it was important GPs communicated this with their patients.
"Work is in progress to reduce turnaround times, with revised testing guidance from the Ministry of Health underway, and by increasing laboratory capacity," she said.
