MPs in Kalsakau's Union of Moderate Parties met on Tuesday with the party executive after Tanna MP Robin Kapapa announced he was leaving the coalition government.
The UMP president Serge Vohor was understood to have wanted the party's MPs to follow Kapapa and also defect.
Local media reports that MPs agreed at the meeting to retain the status quo of having five MPs in government and one in opposition.
But Kalsakau said the six MPs would stick together.
"I spoke to him (Kapapa) yesterday evening after the meeting and he assured me that was of the same view that I had two days ago that he would fall back in line and return to government."
Kalsakau said the UMP is committed to the Bob Loughman-led government for the long haul.
Kapapa's move to join the opposition appeared to take it to a number of 24 MPs within the 52-seat parliament.
The opposition leader Ralph Regenvanu leads the Graon mo Jastis Pati, which has more MPs than any other single party with nine.