WELLINGTON (REUTERS) - New Zealand recorded 21 new cases of Covd-19 on Saturday (Aug 21), with Prime Minister Jacinda Ardern warning that the number will go higher in the next few days and urging the nation to adhere to the lockdown rules.

The infections in the community outbreak of the highly transmissible Delta variant now total 51. Of the 21 new cases, 18 are in the largest city, Auckland, and three in the capital, Wellington.

Ms Ardern on Friday extended the lockdown for the country of 5.1 million until midnight on Tuesday as the outbreak widened beyond the two key cities.

"One of the things that we've learned from New South Wales is that the virus can continue to spread during lockdown," Ms Ardern said of an outbreak in neighbouring Australia's most-populous state.

New South Wales on Saturday logged a record 825 cases as a Delta outbreak surges despite a weeks-long lockdown.

"No one wants an extended lockdown and no one wants to see that rate of transmission, so the No. 1 thing everyone can do right now is reduce down your contacts," Ms Ardern said.

New Zealand's outbreak has been linked to a traveller from Sydney, the capital of New South Wales.

New Zealanders had been living virus-free and without curbs until Ms Ardern on Tuesday ordered what was initially a three-day nationwide lockdown and a seven-day shutdown for Auckland, after the discovery of the first case since February.

The country has recorded just over 2,600 confirmed coronavirus cases and 26 Covid-19 deaths, according to its Health Ministry.

Only 19 per cent of the population has been fully vaccinated, the slowest pace among the wealthy nations of the Organisation for Economic Cooperation and Development grouping.