Here's the MATS reference:
6.2.6.6 When being vectored at night, an IFR aircraft other than a HEAVY jet
aircraft as described at 6.2.6.4, may be assigned a visual approach at any distance
from an aerodrome if:
a. the aircraft has been assigned the MVA; and
b. the aircraft is given heading instructions to intercept final or to position
the aircraft within the circling area of the aerodrome; and
c. the following phraseology is used to assign the visual approach:
1. “WHEN ESTABLISHED IN THE CIRCLING AREA CLEARED
VISUAL APPROACH”; or
2. “WHEN ESTABLISHED ON THE VASIS/GLIDEPATH CLEARED
VISUAL APPROACH”.
The reason for the continued vector may have been that the ATC had more control of your track with regards to separation in case of departures etc. Don't know. Did it hold you up at all or was the positioning reasonable for your aircraft type and profile?
When I was in Darwin (4 years ago) the Radar LSALT was 1500ft to the west, and 1600ft to the east (straight down the centreline of 18/36). May have changed now (hills growing....?)
Cheers,
NFR.