Auckland and Northland temperatures can vary from day to day. If we have had a run of warm and sunny days, the sea temps will warm up. More so when the tide comes in across the warmed sands creating a flow-on effect. Late afternoon high tide is a good time to swim in this respect.
The Bay of Islands receives the benefit of a warm current which flows across the top of the country before it moves south towards Auckland and on to the Coromandel. But depending on where you eventually enjoy the seaside, let's hope our summer is a good one for you and your family.
Hobbiton near Matamata, and not far to drive from Rotorua may interest you? Mt Maunganui is another great beach choice, with its well-developed facilities and warm bathing pools at the foot of the mount/Maunga.