Speaking at Auckland City Hospital this afternoon, Bloomfield said there are no new cases of Covid-19 in New Zealand, for the fifth day in a row.
Yesterday around 100 tests were carried out, and labs were equipped to do 500 tests a day if needed. However, as of today there are no additional probable cases.
Only one patient has required hospital-level care so far, but they have since been discharged. All five are now in isolation at home.
Bloomfield said all close contacts of those cases remain in self isolation and are being followed up daily by health staff.
He said close contacts of the five cases had been "very compliant" in self-isolating.
There was a high degree of trust for those people who were self-isolating coming from countries facing outbreaks, but the ministry was looking at other ways to enforce those self-isolation requirements if necessary, including legal options.
The Ministry of Education is also planning for the possibility of shutting down schools.
Bloomfield reiterated yesterday's advice that anyone who was at all unwell should not go out and put others at risk.
That was particularly important given there were concerts and large gatherings coming up including Polyfest and the Christchurch mosque shootings memorial.
This morning's pandemic announcement formalises the approach that is already being taken, Bloomfield said.
"We have been acting and planning as if this would be a pandemic for some weeks and now and we continue to plan and respond with pace."
He said that was that was in line with the World Health Organisation's advice.
New Zealand's influenza pandemic plan follows six phases, dubbed Plan for it; Keep it out; Stamp it out; Manage it; Manage it (post-peak); and Recover from it.
But the approach to Covid-19 is a little different, Bloomfield said. The current approach is "keep it out, stamp it out, slow it down."
Tools like school shutdowns that would normally be used later in a pandemic could be used "preemptively" to help stamp out or slow the spread of Covid-19.
Epidemiologists have suggested 40 - 60 per cent of the world's population could be infected in a worst-case scenario, if the virus was not controlled. In New Zealand's case - if health authorities took no action to contain the virus that could mean millions of people could be infected.
Bloomfield said there had been figures "bandied around" but authorities were working hard on the combined "keep it out, stamp it out, slow it down" approach to avoid such a scenario.
"We've been successful to date and believe me we are very focused on identifying what we need to do to ensure we don't end up with that situation of a widespread community outbreak," he said.
By Wednesday there were more than 9,000 people or households which had started or completed the self-isolation process.
Among them were 54 North Shore Hospital staff who had been exposed to one of the patients considered a "probable" case of Covid-19.
They were returning to work progressively from Wednesday and would all be back at work by next Monday, assuming they remained well.
Dr Dale Bramley, lead CEO for the northern region DHBs, said there were several arms to their response. Auckland's DHBs have this week increased their presence at New Zealand's borders.
There have been 252 close contacts actively followed up to date.
Hospitals are also being very careful about responding to people who turn up in the ED with symptoms - staff are using protective gear and patients are being isolated.
Bramley said a response team from all three DHBs, with some assistance from Northland, had been pulled together. Some of the duties they were doing have been picked up by other staff.
The DHBs were currently "managing fine" in terms of staffing levels but were very aware that there would be more pressure on hospitals during the peak winter period.
More than 118,000 people have been infected worldwide and almost 4300 have died, according to the WHO.
