Leicester Tigers centre Porter wins a first cap, with scrum-half van Poortvliet starting for the first time after a tryscoring debut in the opening Test last weekend.
England will look to bounce back after a 30-28 defeat in the opening game of the three-Test series, also give a debut to Northampton’s Tommy Freeman at Brisbane’s Suncorp Stadium.
The only change in the forward pack is a call-up for Sam Underhill in place of Tom Curry whose tour was ended by concussion.
Head coach Eddie Jones said: “We have decided to make changes in our backline, they were tough calls but we feel they will strengthen the team. There are opportunities to attack against Australia which we can take.
“This game is the perfect practice for the World Cup and our focus is putting on our best performance.”
Australia v England is an 11.00pm kick-off on Saturday, with live coverage on Sky Sports and BBC Radio 5 Live.
15 Freddie Steward (Leicester Tigers, 11 caps)
14 Jack Nowell (Exeter Chiefs, 40 caps)
13 Guy Porter (Leicester Tigers, uncapped)
12 Owen Farrell (Saracens, 95 caps)
11 Tommy Freeman (Northampton Saints, uncapped)
10 Marcus Smith (Harlequins, 11 caps)
9 Jack van Poortvliet (Leicester Tigers, 1 cap)
1 Ellis Genge (Bristol Bears, 37 caps)
2 Jamie George (Saracens, 67 caps)
3 Will Stuart (Bath Rugby, 21 caps)
4 Maro Itoje (Saracens, 57 caps)
5 Jonny Hill (Sale Sharks, 13 caps)
6 Courtney Lawes (C) (Northampton Saints, 94 caps)
7 Sam Underhill (Bath Rugby, 28 caps)
8 Billy Vunipola (Saracens, 62 caps)
16. Luke Cowan-Dickie (Exeter Chiefs, 35 caps)
17. Mako Vunipola (Saracens, 68 caps)
18. Joe Heyes (Leicester Tigers, 3 caps)
19. Ollie Chessum (Leicester Tigers, 3 caps)
20. Lewis Ludlam (Northampton Saints, 12 caps)
21. Jack Willis (Wasps, 3 caps)
22. Danny Care (Harlequins, 85 caps)
23. Henry Arundell (London Irish, 1 cap)