paragraps (5) & (6) of Section 142 cover what you are asking.
As follows:5) The Prime Minister–
(a) shall be dismissed from office by the Head of State if the Parliament passes, in accordance with Section 145 (motions of no confidence), a motion of no confidence in him or the Ministry, except where the motion is moved within the last 12 months before the fifth anniversary of the date fixed for the return of the writs at the previous general election; and
(b) may be dismissed from office in accordance with Division III.2 (leadership code); and
(c) may be removed from office by the Head of State, acting in accordance with a decision of the Parliament, if the Speaker advises the Parliament that two medical practitioners appointed by the National Authority responsible for the registration or licensing of medical practitioners have jointly reported in accordance with an Act of the Parliament that, in their professional opinions, the Prime Minister is unfit, by reason of physical or mental incapacity, to carry out the duties of his office.
(6) The Prime Minister may be suspended from office–
(a) by the tribunal appointed under an Organic Law made for the purposes of Section 28 (further provisions), pending an investigation into a question of misconduct in office within the meaning of Division III.2 (leadership code), and any resultant action; or
(b) in accordance with an Act of the Parliament, pending an investigation for the purposes of Subsection (5)(c), and any resultant action by the Parliament.
Dammit! I am getting to know more about PNG's Constitution than ours(Aust). But I gues we do not have as many problems in that area.