Complex sentence: He went on a sea voyage for his health (possibly he had tuberculosis) and, according to a letter to Will Lawrie dated 25 January 1902, arrived in Wellington, New Zealand in December 1901, travelling at once on to Lyttelton and Christchurch.
Split into simpler sentences: He went on a sea voyage for his health (he may possibly have had tuberculosis). According to a letter to Will Lawrie dated 25 January 1902, he arrived in Wellington, New Zealand in December 1901, and travelled at once on to Lyttelton and Christchurch.