Does this hold true even today, with AFL teams in Sydney, Brisbane and the Gold Coast, and an NRL team in Melbourne?
Out of the 18 AFL teams, four of them are north of the Barassi Line - Sydney, GWS, Brisbane, and Gold Coast. Let's look at the common metrics of popularity used for sports teams to see how they fare:
Average home crowd in 2019 
- Richmond (VIC), 59,987
- Collingwood (VIC), 58,975
- West Coast (WA), 53,513
- Essendon (VIC), 47,733
- Carlton (VIC), 46,730
- Adelaide (SA), 44,514
- Fremantle (WA), 40,896
- Port Adelaide (SA), 33,950
- Geelong (VIC), 33,405
- Hawthorn (VIC), 31,077
- Sydney (NSW), 31,070
- Melbourne (VIC), 28,968
- Western Bulldogs (VIC), 26,747
- St Kilda (VIC), 25,401
- Brisbane Lions (QLD), 24,741
- North Melbourne (VIC), 20,808
- Greater Western Sydney (NSW), 12,267
- Gold Coast (QLD), 11,417
All four teams above the Barassi line are in the bottom half of the competition for attendance, including three of the four worst-attended clubs in the league. Even Sydney, one of the most consistently strong teams over the last decade and a half, is significantly behind the league average attendance of 35,122.
Notably, Gold Coast's highest ever "home" crowd actually occurred in Perth, as part of a scheme where they hosted a "home" game interstate while their actual home ground was being renovated for the 2018 Commonwealth Games.
AFL-audited membership numbers for 2020. Some clubs self-report much higher membership figures by including things like their email lists, but this list is for paying memberships only. 
- West Coast (WA), 100,776
- Richmond (VIC), 100,420
- Collingwood (VIC), 76,862
- Hawthorn (VIC), 76,343
- Carlton (VIC), 67,035
- Essendon (VIC), 66,686
- Geelong (VIC), 60,066
- Adelaide (SA), 54,891
- Fremantle (WA), 51,577
- St Kilda (VIC), 48,588
- Sydney (NSW), 48,322
- Port Adelaide (SA), 46,820
- Melbourne (VIC), 40,571
- Western Bulldogs (VIC), 38,876
- North Melbourne (VIC), 38,667
- Greater Western Sydney (NSW), 30,841
- Brisbane Lions (QLD), 29,277
- Gold Coast (QLD), 16,236
This is even worse reading for the clubs north of the Barassi Line. Sydney is in the same spot and still a fair chunk off the league average of 55,159, but the other three clubs occupy the lowest three spots on the leaderboard, and by a very large margin. And this is despite GWS and Gold Coast both recording club record membership figures this year, in a year that had a slightly more than 6% downturn league-wide (likely due to economic hardship)!
The NRL is much less interesting to look at, with only one team south of the Barassi Line, Melbourne Storm. Melbourne does admittedly draw very good crowds (18,230) and membership numbers (22,221) - 4th in the league for each - but both of these figures are lower than the worst Victorian-based team in the AFL, despite 10 AFL teams splitting the attention in Victoria, while Melbourne Storm has the Victorian NRL market all to itself.  
It's also telling that this there's more clubs in a different country (New Zealand) than in NT, WA, SA, and Tasmania combined. In the 1990s the Adelaide Rams went defunct after two seasons in the NRL, and the Western Reds (based in Perth) lasted only three. I live in Adelaide, and anecdotally I can tell you that most people here are barely even aware that the NRL exists, let alone that we ever had a team participating in it.
League versus Union
is it true that for most rugby fans in Australia, rugby league would be more popular than rugby union, and that the latter is only really followed during the World Cup and other similar international events?
Yes, but with slightly more of an asterisk.
It's unequivocally true that League is more popular in Australia as a whole, and you're right that Union typically only receives mainstream coverage during the Rugby World Cup. Unfortunately I'm not quite familiar enough with Union to get the most reliable sources for this, but most of the dodgy sites I'm finding with Google seem to suggest that League has about 3x the participation and viewership of Union in Australia as a whole.
However, you may see minimal differences outside of the eastern seaboard, particularly in Perth where the Western Force participated in the Super 14/Super Rugby competitions for over a decade during a period where there was no local presence in any League competitions of note.
 AFL Tables - Crowds 2019
 The Age - AFL club membership tally falls for first time in 20 years
 RL Tables - Attendances 2019
 League Unlimited - 2020 NRL Membership Tracker