Background ..
(a) 400 ft is the certification minimum and, subject to terrain and other analysis considerations, there is no reason why this can't be used.
(b) in practice, if the operator runs the sums to optimise for all runways of interest, operating crews would end up with a significant variation of takeoff procedures and third segment heights.
(c) it follows that, for a generally small penalty, there are SOP advantages in grouping runways so that crews have (ideally) one (or a few) "standard" third segment(s) other than for a few special runways which require higher third segments for terrain optimisation.
(d) the upper (maximum) third segment height will be limited by engine time limits for operation at takeoff power/thrust. In some cases there may be other system limits - eg the Dart is limited to a 600 ft third segment for a feathering restriction as I recall from the dim depths of the memory banks.
The standard seems to be 1500' AAL, but why is this
Subject to the previous comments, your operator has chosen to make 1500 ft the SOP operation for a reason which you could establish via a quick question to your flight standards group. Two common drivers are
(a) terrain - the highest runway obstruction consideration is used to dictate the general third segment height. As you are in Oz you may be interested to know that, in the past, CBR 17 was the driver in JT8 days.
(b) with the increased use of a 10 minute thrust limit for operation at takeoff power/thrust, an operator may well elect to use the highest capability to minimise flight standards worries with low level failures etc.
what is the restriction to stop you doing it at a minimum of 400' like a regular take off?
Generally only terrain in the segments. 400 ft presupposes that the terrain is benign.
Keep in mind that the third segment can run for a long distance, especially on twins and, with the fourth segment added in, you can have a LONG distance to the 1500 ft net point. By pushing the third segment higher the overall engineering analysis workload can be cut back significantly.