* 12 new confirmed cases and one probable case
* Two cases outside Auckland city
* Ardern to announce decision on lockdown (Adds details from news conference by NZ Health Minister and Director General of Health)
By Praveen Menon
WELLINGTON, Aug 14 (Reuters) - New Zealand's first coronavirus outbreak in three months has spread further, officials reported on Friday, just hours before Prime Minister Jacinda Ardern is due to announce whether a lockdown in the country's biggest city will be extended.
Officials reported 12 new COVID-19 cases on Friday, all linked to a now 30-strong cluster that was first detected in a family in Auckland two days ago. Officials believe an imported strain of the virus is responsible for the country's first outbreak in three months, but are still investigating how the family was infected.
Ardern, under pressure ahead of a general election next month, repeated her "go hard, go early" response to the pandemic this week, putting Auckland, home to about 1.7 million people, into lockdown and reinstating social distancing measures across the country.
Ardern is due to announce later on Friday whether those measures will be extended, but there is growing concern that a repeat of the tough five-week lockdown she imposed earlier in the year could cripple the economy.
Director General of Health Ashley Bloomfield said it was a positive sign that all the new confirmed cases were linked to the Auckland cluster, but noted two were recorded in Tokorua, in the neighbouring Waikato region. Authorities had also identified one other probable case, Bloomfield said, in which connections still had to be traced.
"We are not out of the woods yet," he said during a televised news conference, adding that contact tracing and testing would continue at high levels in coming days.
Health Minister Chris Hipkins said genome testing suggested the new virus outbreak had originated in Britain or Australia, but officials were still investigating how the family in Auckland contracted it.
Bloomfield dismissed suggestions by health experts it was likely the virus had been quietly spreading in Auckland for weeks, saying there was "very good evidence" that was not the case.
"The nature of this outbreak shows how once you identify the first case you find quite a lot quite quickly," he said. "We just wouldn't have not found cases in community if it was lurking away in the community."
Deputy Prime Minister Winston Peters said late on Thursday that a quarantine facility breach had been identified as the source, but Hipkins said there was no evidence for that yet.
The main opposition National Party has blasted the government, saying it failed to secure quarantine facilities and accusing it of withholding information about the latest outbreak.
New Zealanders celebrated when Ardern appeared to eliminate community transmission of the coronavirus with the earlier hard lockdown that forced almost everyone to stay at home.
But opinion is divided on whether the 40-year-old leader should repeat that strategy, given its huge economic cost and mounting global evidence that the virus cannot be permanently suppressed.
Westpac Banking Corp estimated the current level of lockdown measures in Auckland and the rest of New Zealand would cost the economy about NZ$300 million, or 0.5% of gross domestic product.
Reserve Bank of New Zealand Deputy Governor Geoff Bascand told Reuters a sustained resurgence of the virus posed "a major risk" to the bank's outlook, given its baseline scenario has an assumption that the virus is contained in the country.
Ardern is expected to announce her decision on lockdown measures at 1730 local time (0530 GMT) after meeting with her cabinet and the release of the daily infection numbers. (Reporting by Praveen Menon; Editing by Jane Wardell)