In 1862, Forsaith entered the services of the church.
Since 1850, he had belonged to the Presbyterian Church.
In early 1865, he was considered to become a minister at the gold fields in Tuapeka, but the Presbytery in Dunedin voted against licensing him, as he hadn't completed his studies yet.
The view was held that a "minister of the Gospel should be able to read at least the New Testament in the original [Greek] text".
Instead, Forsaith was offered a missionary post to the gold fields, which he declined.
In July of that year, he was instead ordained as a pastor at the new Congregational Church at Port Chalmers.

In 1867, Forsaith accepted an invitation for a pastorate at Woollahra.
His health had suffered over the winter, and apparently he hoped for an improvement in the warmer climate.
He left New Zealand on board the Parisian on 23 September 1867.
In 1868, he moved to Parramatta, where he initially held services in the School of Arts.
A church was built for the community, which opened on 19 May 1872.
Also in 1872, Forsaith became chairman of the Congregational Union of New South Wales.
In 1874, he acquired Morton House in Melville St, Parramatta, the house of the solicitor John Morton Gould, father of Albert Gould.
Morton House remained the principal family residence for the rest of his life.

In 1878, he became resident chaplain at Camden College, a theological college founded in 1864.
From there, he initiated a branch mission at Haslam's Creek, but moved back to Parramatta in 1882.

Subsequent to this, a period of travel started.
He went to New Zealand (he left Melbourne for New Zealand in March 1882 on the Rotomahana), America, Canada and Europe, including lecture series in Britain (which attracted many new immigrants to New Zealand) and officiating at the Presbyterian Church in Venice.
He returned to Melbourne from Britain in April 1884 on the Berengaria.
He then relieved at churches in Australia, Dunedin and Invercargill.
In 1862, Forsaith entered the services of the church. Since 1850, he had belonged to the Presbyterian Church. In early 1865, he was considered to become a minister at the gold fields in Tuapeka, but the Presbytery in Dunedin voted against licensing him, as he hadn't completed his studies yet. The view was held that a "minister of the Gospel should be able to read at least the New Testament in the original [Greek] text". Instead, Forsaith was offered a missionary post to the gold fields, which he declined. In July of that year, he was instead ordained as a pastor at the new Congregational Church at Port Chalmers.
In 1867, Forsaith accepted an invitation for a pastorate at Woollahra. His health had suffered over the winter, and apparently he hoped for an improvement in the warmer climate. He left New Zealand on board the Parisian on 23 September 1867. In 1868, he moved to Parramatta, where he initially held services in the School of Arts. A church was built for the community, which opened on 19 May 1872. Also in 1872, Forsaith became chairman of the Congregational Union of New South Wales. In 1874, he acquired Morton House in Melville St, Parramatta, the house of the solicitor John Morton Gould, father of Albert Gould. Morton House remained the principal family residence for the rest of his life.
In 1878, he became resident chaplain at Camden College, a theological college founded in 1864. From there, he initiated a branch mission at Haslam's Creek, but moved back to Parramatta in 1882.
Subsequent to this, a period of travel started. He went to New Zealand (he left Melbourne for New Zealand in March 1882 on the Rotomahana), America, Canada and Europe, including lecture series in Britain (which attracted many new immigrants to New Zealand) and officiating at the Presbyterian Church in Venice. He returned to Melbourne from Britain in April 1884 on the Berengaria. He then relieved at churches in Australia, Dunedin and Invercargill.
Thomas Forsaith