Would this require constitutional change, ie. a referendum?
No, increasing the ACT Senate representation only requires a legislative change, not a constitutional change, and is therefore easy to amend.
Would the increase in Senators invoke the nexus provision?
Section 24 of the Constitution provides that the number of members of the House of Representatives must be twice the number of senators; however, Senators from the two territories are not counted in determining the size of the House of Representatives. The High Court held in the 1977 McKellar’s Case 1977, that ACT and NT senators are not included in the “nexus” calculation under s24 of the Constitution. Thus increasing ACT Senate representation would not need to lead to an increase in House of representative seats.
Additionally, Section 122 of the Constitution states that the Parliament has power to make laws for the territories ‘on the terms it thinks fit.’ This suggests that the ACT and NT are not required to have identical representation and thus extra Senate seats could likely be granted to one and not the other.
Could this legislation be enacted?
The Commonwealth Parliament has the power to legislate for the representation of territories, per Section 122 of the Constitution. It would be constitutionally valid.