Martin (1992: 450-1):
Where the tonic falls on the final salient syllable in a tone group, the domain of the structural function New is indeterminate; in principle it extend may [sic] leftwards from the tonic syllable until the initial salient syllable of the tone group is reached. Where a tonic falls on other than the final salient syllable of the tone group, then all information following the tonic syllable is Given. In order to simplify the interpretation of New presented here, only the minimal domain of the New will be considered; this will be taken as the highest ranking clause constituent (usually a ranking group or phrase) the tonic syllable falls on the final salient syllable of.
 This is potentially misleading. 'New' is not a 'structural function' of the tone group. Given (optional) and New (obligatory) are elements of the function structure of the information unit. An information unit (lexicogrammatical stratum) is realised by a tone group (phonological stratum). Halliday & Matthiessen (2014: 116):
Each information unit is realised as a pitch contour, or tone, which may be falling, rising or mixed (falling-rising, rising-falling). This pitch contour extends over the whole tone group. Within the tone group, one foot (and in particular its first syllable) carries the main pitch movement: the main fall, or rise, or the change of direction. This feature is known as tonic prominence, and the element having this prominence is the tonic element (tonic foot, tonic syllable). … The element having this prominence is said to be carrying information focus.
The tonic foot defines the culmination of what is New: it marks where the New element ends. In the typical instance, this will be the last functional element of clause structure in the information unit. As this implies, the typical sequence of informational elements is thus Given followed by New. But whereas the end of the New element is marked by tonic prominence, there is nothing to mark where it begins; so there is indeterminacy in the structure. If we take an instance out of context, we can tell that it culminates with the New; but we cannot tell on phonological grounds whether there is a Given element first, or where the boundary between Given and New would be. (This is not always true….)
 It is the tonic foot that marks where the New element ends, not the tonic syllable.
 To be clear, in SFL theory, this is the focus of New information.