Thanks, I just emailed them and got this response if it helps anyone else.
Please be informed that if you hold an unused/unstamped/not validated migrant visa that specifically states that you are permitted to remain in Australia indefinitely / permanently for flights departing from London/Manchester you will then be entitled to our migrant baggage allowance (i.e. 40 kilos per seat paying passenger).
This baggage allowance will only be granted when you check in at the airport when this visa is sighted and checked that it has also not been travelled on before, this has to be the first time it is being used.
You can have as many items of checked luggage as you wish provided:
• The total combined weight of all the items put together does not exceed your total checked allowances
• No single item of baggage exceeds 32 kilos in weight, as a health and safety requirement