From the Qantas website
For tickets issued from 1 December 2010
Allowance
Economy/Premium Economy 23kg (50lb) (Exception: 30kg (66lb) on QF337/338/358/359 to/from Ho Chi Minh only)
Business 32kg (70lb) (Exception: 40kg (88lb) on QF337/338/358/359 to/from Ho Chi Minh only)
First 40kg (88lb)
Qantas Club members 32kg (70lb) in Economy/Premium Economy
42kg (92lb) in Business
50kg (110lb) in First
Silver Frequent Flyers 32kg (70lb) in Economy/Premium Economy
64kg (140lb) in Business
64kg (140lb) in First
Gold Frequent Flyers 46kg (101lb) in Economy/Premium Economy
64kg (140lb) in Business
64kg (140lb) in First
Platinum Frequent Flyers 64kg (140lb) regardless of travel class
and you also need to be aware that Qantas have a different hand baggage allowance to BA based on weight as well as size. My experience is that all hand baggage is weighed at check in by Qantas.
All International
First/Business/Premium Economy:
2 x 115cm (45in) bags or
1 x 115cm (45in) bag plus 1 x 185cm (73in) non rigid garment bag
Economy:
1 x 115cm (45in) bag or
1 x 185cm (73in) non rigid garment bag
7kg (15lb) per piece
Baggage Dimensions
Maximum dimensions of Carry-on Baggage*
Length Height Depth Total
48cm (19in) + 34cm (13in) + 23cm (9in) = 105cm (41in)
56cm (22in) + 36cm (14in) + 23cm (9in) = 115cm (45in)
60cm (24in) + 114cm (45in) + 11cm (4in) = 185cm (73in)
non-rigid garment bag
81cm (31.9in) + 30(11.8in) + 19cm(7.5in) = 130cm (51in)
musical instrument
*Total dimensions are measured by adding together the length, height and depth of the bag.
but if you are just transiting directly in Hong Kong you might get away with the BA hand baggage allowances, and clearly on your return flights you will be OK on BA.