The Holden Racing Team's troubled start to the Supercars season has seen major changes within its engineering department ahead of Hidden Valley.
Speedcafe.com understands that Tander's former engineer Blake Smith has been dismissed from the team while Alistair McVean, who previously worked with Courtney, will remain in another role.
Smith joined the squad from Tekno Autosports in 2014, while McVean is a 12 year veteran at the Clayton, Victoria, based outfit.
It is currently unclear who will take over the race engineering positions in Darwin, although it is expected that the roles will be filled internally.
Poor results have shone an increasingly bright spotlight on the Adrian Burgess-led team, which is out of contract with Holden and both of its drivers at the end of the current season.
Tander and Courtney sit eighth and 11th in the championship respectively after 11 races, scoring just one top five finish between them in the eight races since Adelaide's season opener.
Courtney won the second heat at the Clipsal 500, again showing the team's prowess on street circuits before struggling at other venues.
Qualifying speed has proven the squad's biggest issue, with both cars qualifying 10th or worse for six of the eight races across Symmons Plains, Barbagallo, Phillip Island and Winton.
The struggles reached their zenith last time out at the squad's own test track, where Tander and Courtney qualified 19th and 20th respectively for the Sunday race.
The engineering shake-up comes just five events into a year that has seen the team trim down from four cars to two.
Its off-season restructure also included significant engineering staff changes, with Alex Somerset moving from the team's now defunct fourth car to become technical director.