The Church established the diocese on 4 June 1848 to resolve the difficulty between the Bishop of Auckland (Jean Baptiste Pompallier) and the religious clergy in his diocese. A line roughly across the 39th parallel between Waitara in the west and Wairoa in the east divided the dioceses. Wellington diocese began to the south of the line: it included what would today encompass the dioceses of Palmerston North, Wellington, Christchurch and Dunedin. On 1 May 1850 Bishop Viard, SM, with Marist Priests Jean Forest, Jean-Baptiste Petitjean, Antoine Garin, Jean Antoine Séon and Jean Lampila; eight Marist Brothers, three "Sisters of Mary" and seven others — a group totalling 24 in all — arrived in Wellington aboard the Clara from Auckland.
Within the month Viard had dispatched missionaries to Nelson, Akaroa and the Hutt Valley. He had also purchased two sections on Hill Street; Lord Petre gave him a third. The Hill Street site stood adjacent to what would become the seat of New Zealand's secular government.
Viard opened The Cathedral of St Mary on 7 December 1841 the eve of the feast of the Immaculate Conception. After the 23 January 1855 earthquake devastated Wellington Viard consecrated the diocese to the Blessed Virgin under the title of the Immaculate Conception in order to enlist her protection from further earthquakes. The 700 who packed the cathedral included about 300 Protestants.
Séon at Akaroa had gone to Purau in Lyttelton Harbour to visit when the Charlotte-Jane — the first of the four ships that established the Canterbury settlement — arrived at Lyttelton in 1850. He serviced the needs of the lower South Island in the early years.
Lampila, who had already visited Hawkes Bay from Whakatane, departed on 3 June 1850 with Brothers Basile and Florentin. A fierce storm saw them land further north outside the Diocese and Viard had to redirect them later. They arrived in Pakowhai (near the future Napier) in December. Shortly afterwards, Euloge Reignier replaced Lampila and opened his first church there on 6 March 1859. Basile and Florentin (John) grew the first grapes there. Two roads in the Meeanee area (Basil Road and ??) commemorate them, and Mission Vineyards traces its history to their efforts.
The Church grew somewhat more slowly in Taranaki. Pezant left on 12 May 1850, but on his return five months later admitted that the very few faithful did not yet need a full-time mission. Pezant later began in Wanganui. Lampila joined the mission up the Wanganui River.
In Napier in February 1865 the Sisters of the Congregation of Our Lady of the Missions established their first girls' school outside France. This would develop into Sacred Heart College, Napier. A year later the Church established a school and orphanage for Maori girls nearby. This became Hato Hohepa, Greenmeadows.
Delphin Moreau made several visits to the Otago and Southland regions, carrying on what Pompallier had begun in the earliest days of the Catholic Mission. The 1861 discovery of gold near Tuapeka and Lawrence, then in the following year in Central Otago near Cromwell and Arrowtown, caused a rush, but by this time Aimé Martin had joined Moreau. Miners also panned the Shotover River and the Taieri, and the gold rushes had a significant impact on the area.
Bishop Viard sent several priests to visit the area — as he did himself. In 1868 Viard went to Rome and later attended the First Vatican Council. While in Europe he met with the Marist authorities in Lyon and with Propaganda. Pompallier had arrived in Rome to tender his resignation, and Viard requested assistance for the southern part of his diocese. In 1869 Otago and Southland became a separate diocese, called Dunedin, with Bishop Patrick Moran (an Irishman translated from East Cape Colony in South Africa) as its first Bishop.
Petitjean, Chataigner and Chervier