I have been to Port Lincoln and I had three days to do some of the Investigator Trail. I made a loop of the eastern part, walking 75 kms over the three days in June. I started from the park entrance, walked to Pillie Lake and crossed to Taylor’s Landing, then followed the coast around via September Beach, Cape Donnington, Surfleet Cove, Stamford Hill, Woodcutters Bay and back to the entrance. I got to and from the park by taxi - very straightforward and inexpensive although I had the impression taxis do not often get asked to go there. There was water in every tank I saw but I did not see many tanks. Being winter this was not a problem. I barely saw anyone. I would definitely recommend the loop I did for beautiful beaches and lovely coastal bush.
I saw, but did not walk on, the Parnkalla Trail from the town to the national park, which is largely alongside the road and goes through some industrial areas. It would be ok to walk but not wildly interesting.
I have blogged about my visit here:
https://pinkcapadventures.blogspot.com/ ... ralia.html