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Jonathan (6) and Elizabeth Rapsey (9) set out from Cape Reinga with their parents Chris and Jorinde Rapsey in October, and expect to complete the 3000km walking journey to Bluff in March.
The family arrived in Wellington on Tuesday for a couple of rest days before their ferry to the South Island tomorrow.
''At the start, the children were tired and found some parts hard but now they are loving it,'' Mrs Rapsey said.
''It amazes me that at the end of a long day Jonathan still jumps around full of energy.''
The family was averaging about 20km a day and had even clocked up more than 30km on a few occasions, she said.
Both children began the journey without a pack, but by the time they reached Auckland Elizabeth was adamant she should have one, and has carried it ever since.
Mrs Rapsey said the trip had gone smoothly.
''There have been some arguments over which flavour of muesli bar they are having, but overall they have just been really happy.''
She and her husband were inspired to walk the trail after venturing into Fiordland with the children twice in the first half of the year and being amazed at how well they did on the Hollyford Pyke Loop and a five-day return tramp to Monowai Hut.
The family are renting out their house to help fund the trip, and the children have been able to keep up their home schooling.
Mrs Rapsey was confident they would complete the trail and was looking forward to spending Christmas Day on the Queen Charlotte Track, ''with some extra yummy food''.
Te Araroa Trust chief executive Mark Weatherall believed the youngest person to have completed a single island was 11-year-old Cedric Wittwer, of Auckland.
He was ''thrilled'' to hear of the Rapsey family's journey.
''If these guys can do it, then so many other Kiwis can do it, too.''
Mr Weatherall said more than 1000 people were expected to walk the full length of the trail this summer.
The number of New Zealanders walking the trail was also on the rise.
About a third of all through-walkers were from New Zealand, compared with about one in five in 2016-17.