@Aussiebird Hi Aussiebird, I'm a long-time lurker here. I can offer you some information about extensions to entry dates, because we are in a somewhat similar position. We have until Sept 4th to activate my husband's 309/100. I contacted our migration agent in early May, then my husband rang Home Affairs twice in June to ask for an extension. We asked for special consideration - I won't go into all the ins and outs of that - but we had good and valid reasons for asking. The answer was no.
The explanation given by my agent was as follows. Once the visa is issued/granted, it's pretty much set in stone. There is no mechanism under the law to change its conditions and that includes the Must Enter By Date. However, a breach of the Must Enter By date does not necessarily result in automatic cancellation. It only makes the visa “liable” for cancellation should DHA wish to do so.
So if you can't enter by your Must Enter By date, all you can really do is seek "forgiveness" after the fact. But after all the hassle, tears, and money you have to spend to get the visa in the first place, who wants to leave it to DHA's discretion to decide whether or not they'll let you in after the deadline has passed? I didn't even bother trying to work out what the procedure would be. We decided to go ahead and fly out there rather than have to deal with DHA again.
You can try calling Home Affairs on +61 13 18 81 as we did. They will tell you to read the web site, etc. etc., which we told them we had already done. Just remain politely insistent that they hear you out. I told them that on the COVID-19 page on their web site there are ways for people to apply for exemptions if they want to leave Australia, but nothing for those who want to delay their arrival. I wish DHA would realise that it's in the national interest, both in terms of the risk to public health and the taxpayer money spent on quarantine, to permit those of us who wish to delay entry to do so. We've been so careful to lockdown and do the right things during the pandemic. Now here we are, having to travel to Australia and take risks just so we can tick the visa box.
I'm sorry I can't give you better news and I wish you the best of luck.