Section 4: Special defences under sections 1 and 3.

(1) Where a person is charged with an offence under section 1 of this Act, or is charged with an offence under section 3 of this Act as the owner or master of a vessel, it shall be a defence to prove that the oil or mixture in question was discharged for the purpose of securing the safety of any vessel, or of preventing damage to any vessel or cargo or of saving life:
Provided that a defence under this subsection shall not have effect if the court is satisfied that the discharge of the oil or mixture was not necessary for the purpose alleged in the defence or was not a reasonable step to take in the circumstances.
(2) Where a person is charged as mentioned in subsection (1) of this section, it shall also be a defence to prove-
(a) that the oil or mixture escaped in consequence of damage to the vessel, and that as soon as practicable after the damage occurred all reasonable steps were taken for preventing, or (if it could not be prevented) for stopping or reducing, the escape of oil or mixture; or
(b) that the oil or mixture escaped by reason of leakage, that the leakage was not due to any want of reasonable care, and that as soon as practicable after the escape was discovered all reasonable steps were taken for stopping or reducing it.
(3) Where a person is charged with an offence under section 3 of this Act as the occupier of a place on land, or as the person in charge of any apparatus, from which oil or a mixture containing oil is alleged to have escaped, it shall be a defence to prove that the escape of the oil or mixture was not due to any want of reasonable care, and that as soon as practicable after the escape was discovered all reasonable steps were taken for stopping or reducing it.
(4) Without prejudice to subsection (3) of this section, it shall be a defence for the occupier of a place on land, who is charged with an offence under the last preceding section, to prove that the discharge was caused by the act of a person who was in that place without the permission (express or implied) of the occupier.
(5) Where a person is charged with an offence under section 3 of this Act in respect of the discharge of a mixture containing oil from a place on land, it shall (without prejudice to any other defence under this section) be a defence to prove-
(a) that the oil was contained in an effluent produced by operations for the refining of oil;
(b) that it was not reasonably practicable to dispose of the effluent otherwise than by discharging it into waters to which the last preceding section applies; and
(c) that all reasonably practicable steps had been taken for eliminating oil from the effluent:
Provided that a defence under this subsection shall not have effect if it is proved that, at a time to which the charge relates, the surface of the waters into which the mixture was discharged from the place in question, or land adjacent to those waters, was fouled by oil, unless the court is satisfied that the fouling was not caused, or contributed to, by oil contained in any effluent discharged at or before that time from that place.
(6) Where any oil, or mixture containing oil, is discharged in consequence of-
(a) the exercise of any power conferred by sections 273 and 275 of the Merchant Shipping Act (which relate to the removal of wrecks by the Receiver of Wreck);
(b) the exercise, for the purpose of preventing an obstruction or danger to navigation, of any power to dispose of sunk, stranded or abandoned vessels which is exercisable by the harbour authority,
and apart from this subsection, the person exercising the power under the said sections 273 and 275 or, as the case may be, the harbour authority exercising the power, (or a person employed by or acting on behalf of that person or authority) is guilty of an offence under section 1 of this Act or under section 3 of this Act, in respect of that discharge, the authority or person shall not be convicted of that offence unless it is shown that it or he failed to take such steps (if any) as were reasonable in the circumstances for preventing, stopping or reducing the discharge.