I’m not an engineer, but I don’t think you would be able to tunnel underneath the Union Complex / other buildings on the depth that would be available leaving UQ Lakes. My understanding is that the Adelaide Street tunnel works at shallow depth because it follows the street.
With the flooding, BCC Floodwise Report gives a 1% AEP / 100 ARI (i.e. 1 in 100 chance each year) level of 8.3m AHD and Jan 2011 is shown as being 7.9m AHD. For comparison, the Brisbane Metro Cultural Centre underground station was designed with a 150 ARI immunity but would be “inoperable” at 75 ARI. The existing UQ Lakes station is between 6m and 8m AHD so would also close at a similar flood extent, even if the tunnel is designed for greater immunity.