Initially police said 400 people were at the wedding, but later revised the number to 150.
Weddings are currently allowed only in "exceptional circumstances" in the UK - such as one partner being dangerously ill - and with a maximum of six people attending.
Police said the organiser could face a £10,000 (NZ$19,051) fine. Many guests fled as police arrived, but five people who attended received £200 fines.
The Yesodey Hatorah Senior Girls' School, a state-funded Orthodox Jewish high school, said in a statement that it was "absolutely horrified about last night's event and condemn[s] it in the strongest possible terms".
The school said its hall had been leased to an outside organisation and "we had no knowledge that the wedding was taking place".
UK Chief Rabbi Ephraim Mirvis condemned the rule-breaking event.
He tweeted: "At a time when we are all making such great sacrifices, it amounts to a brazen abrogation of the responsibility to protect life and such illegal behaviour is abhorred by the overwhelming majority of the Jewish community."
This is a most shameful desecration of all that we hold dear. At a time when we are all making such great sacrifices, it amounts to a brazen abrogation of the responsibility to protect life & such illegal behaviour is abhorred by the overwhelming majority of the Jewish community. https://t.co/pSTrVmFPu8
The school's principal, Rabbi Avrohom Pinter, died in April after contracting the coronavirus.
Pubs, restaurants and entertainment venues in Britain are closed, and people are required to stay largely at home, as part of restrictions to curb a new surge in the virus.
The UK has recorded more than 95,000 Covid-19 deaths, the highest toll in Europe.