So... You are not permitted to use your car to drive to the beach and it is strongly suggested that you should not enter the water.
So there is no direct open statement saying it is forbidden. However as below from the water safety nz strongly suggested that you dont. If you are not within walking distance of the beach then you are screwed!
Water Safety New Zealand (WSNZ) is urging all New Zealanders to recreate responsibly during the lockdown period and stay out of the water.
While it might seem tempting to head out to sea, the rivers, lakes or head for the hills for some remote recreation, the public is reminded that New Zealand’s water safety and search and rescue services are predominantly made up of volunteers who will also be in lockdown. WSNZ supports the positions of all its’ partner agencies who are united in their messaging.
“The message is a clear one,” says WSNZ CEO Jonty Mills. “If you get into trouble you will put others at risk who will have to come out of self-isolation to help you and potentially be exposed to COVID-19.”
New Zealanders are asked to listen to the authorities and avoid high risk activity during this period that could potentially end up needing rescue or emergency services.
“The Prime Ministers advice is clear. Stay at home to save lives. We’re right behind that resolve.”