There is a new walking trail to get there. It’s about four to five hours. This is from the NZ Herald [source]:

What is billed as being one of New Zealand’s best walks will open this weekend, with upwards of 30,000 hikers per year expected.
The Tarawera Trail will make the favourite picnic and camping spot of Hot Water Beach on the edge of Lake Tarawera now available for walkers and hikers – albeit with a little effort.

The collaborative project has taken over nine years of negotiations and development, with 11km of trail from the Te Wairoa car park to Te Rata Bay (Hot Water Beach) now complete. The one-way walk is expected to take moderately fit walkers around 4-5 hours to complete.

The Tarawera Trail was built in partnership between the Department of Conservation and several private Maori land blocks that border Lake Tarawera and the Te Wairoa Valley.

Tarawera Trail Trust trustee Te Ohu Mokai Wi Kingi says the opening of the trail is a great asset for Rotorua with the trail set to become one of New Zealand’s greatest walks.

“The volcanic backdrop, geothermal features of the lake, surrounding bush and cultural and historic stories make this trail one of a kind.

“We’re expecting 33,000 people a year to walk the trail – or around 90 people per day.”
Panels along the trail will educate walkers and hikers about the history and ecology of the area.
“The trail is a perfect addition to the region as it has a low impact on the environment and ensures tangata whenua, as kaitiaki of the area, continue the stories of their forefathers.”
In October this year the trail was honoured by the gift of a carved wooden Pou, Ohomairangi, by Tourism New Zealand and Te Puia|NZMACI, acknowledging the local hapu, Ngati Hinemihi and Tuhourangi’s significant contribution in establishing New Zealand’s tourism industry.

Ngati Hinemihi and Tuhourangi were New Zealand’s very first hosts, welcoming and guiding domestic and international visitors to the Pink and White Terraces on Lake Tarawera.

The official opening powhiri and ribbon cutting is on December 14 with the trail open to walkers and hikers on 15 December.

The Tarawera Trail is the first of the proposed network of trails, planned for development in the future, involving DOC and Maori private land owners to connect the 14 Te Arawa Lakes in this area.