Martin (1992: 193-4):
Alongside being modulated through obligation, causal relations may be modalised. With both condition and purpose the relation between Cause and Effect is a contingent one; and in both cases the Effect is irrealis — there is a possibility, a probability or a certainty that it will be determined by the Cause, but as the meanings are made it has not yet ensued. This is the opposition between [4:57] and [4:58-9].
CONSEQUENCE (Effect realis)[4:57] Cause Because we trained hard,Effect we won.
CONDITION (Effect irrealis)[4:58] Cause If we'd trained hard,Effect we'd have won.
PURPOSE (Effect irrealis)[4:59] Cause We trained hard,Effect so that we'd win.
 This confuses enhancement (ideational metafunction) with modality (interpersonal metafunction). Cause is language in the rôle of construing experience, whereas modalisation is language in the rôle of acting on each other. The probability of a construed causal relation is expressed using the interpersonal system of modalisation, as in genes possibly/probably/certainly determine behaviour. The (interpersonal) modalisation values are not features of (ideational) causal relations.
 Neither condition nor purpose can be logically construed as cause and effect. The logical meaning of condition is 'if P then Q' and the logical meaning of purpose is 'because intention Q so action P'. Cause and effect, on the other hand, is the logical meaning of reason and result: 'because P so result Q'. For why conditional statements are not statements of causality, see also here.
 The logical relation here is reason, not consequence. This is shown by the expression of the logical relation in the dependent clause ('because'), just as condition is shown by the expression of the logical relation in the dependent clause ('if') and purpose is shown the expression of the logical relation in the dependent clause ('so that').
The use of the term 'consequence' here is inconsistent with its use in both SFL theory and formal logic, where it is a term used for conditional — not causal — relations.