How much is an appropriate amount to request for a rental reduction due to serious mould issues?

I have a friend who rents a one-bedroom apartment in Sydney who has had a serious mould issue on the walls and ceiling of her bedroom for the last 2-3 months. She reported it to to the real estate agent almost 9 weeks ago and while they are trying to deal with it now, it took a good 4-5 weeks before there was much action. She is unable to use her mattress  (purchased less than a year ago) as it appears the mould has moved into it and she has been sleeping on her couch since reporting it. She has had to move all her clothes out of the built in wardrobe as the mould has spread there. Most of her bedroom has been moved into her living room so she is not getting the full benefit of the unit she rents. Other units on her side of the building are experiencing similar issues and building management are looking into it (albeit slowly).

At the 7 week point, realising this was going to take longer to sort out than anticipated, she requested a rental reduction, backdated by 4 weeks and ongoing until the mould issue is resolved. We looked into this but couldn’t find any guidance on how much of a reduction to request. She currently pays $370 per week and given she is not able to use her bedroom at all and her living room is now turned into a makeshift bedroom so she can’t really entertain and have friends over, she suggested a $150 per week reduction. The real estate agent has baulked at this and indicated that her rent is already around $150 below market rate. After pointing out that what she pays in rent and the request for a rental reduction are two separate things (clearly my friend will be receiving a rental increase at the end of all this!), she is now waiting for the owner to come back with an alternative amount. The owner is waiting until they receive a report from building management as to the extent of the damage and what it will take to fix it. This is clearly going to take some time and my friend will of course continue paying her usual rent until she gets some sort of resolution. The owner has suggested she can move out (she is off lease but has lived in the property for many years) but my friend doesn’t want to leave, she likes the unit and it’s location. 

Does anyone have any guidance around what would be a suitable rental reduction in this situation and what is a reasonable period of time to wait for the owner to make a decision around applying the reduction? 

Thanks

Tracey