No worries !
I think with 250k, you can negotiate with the branch manager. What I'd do is to get a benchmark using XE.com middle rate and talk to them
For example AUD/MYR current Xe.com rate is 2.9437.
For TransferWise rate, it is 2.9203
So, the spread (ie profit) for TW is 0.0234, or 0.79%
You can negotiate with NAB to get lower than that since it is a large amount. For Malaysia bank transfer money out, I can get about around 0.4% spread for larger amount like RM350k.
My observation here in Malaysia is for smaller amount (below RM350k), fintech companies have better rates. For larger amount, banks have better rate after negotiation. I don't have experience transferring large amount back to malaysia though, so I am not sure whether the banks like NAB can match the rates
Another thing to consider is if you use TW you have to transfer the amount in stages, as there is a daily limit (online banking daily limit, as well as TW max daily limit). If your limit is A$20k per day, then you have to do it over 13 days

The big 4 banks are not competitive at all - i have asked them previously and they were not happy to entertain me as they view a couple of hundred grand (Aussie) as not a significant amount. I may check with HSBC but i suspect the discount will not be very significant. The big banks here, i feel, is intentional on not competing against each other to maximise on their profits.