I got an email from HSBC, and learnt that this is a part of HSBC's home&Away Privilege Program. With this deal, Platinum Membership (allows access to Sydney-city FF gyms) is $20.80 a week (18-mth contract) ($41.60 per fortnight). For the 12 month contract it is $22.40 per week. I signed up last week when they also had a 4 weeks free offer going (since ended, sorry), and my experience with the Customer Service was that it was actually pretty good.
The 20% off is also available for Passport membership or Home memberships apparently.
I'd strongly recommend you making sure you read through the T&C before you sign any contract (14 day cooling off period generally applies), and make sure you understand all charges/rules etc and have this all filled out before you sign.
HSBC Home&Away Privilege Program page: http://www.hsbc.com.au/1/2/homeandaway