I seem to be able to download as reasonable speed, but as I am now doing a lot of videoconferencing, I am finding my NBN internet is not doing upload fast enough - other users in my videoconference report a lag when trying to see and hear me
Which type of NBN are you on? What is your plan?
Are you doing video-conf from a computer connected by cable or by wifi device?
Wifi will get faster the closer you are to the router.
Do you use 2.4G or 5G band? Changing wifi channel might help.
Other wifi devices can cause slow response.
Other networks in the neighbourhood can cause interference. Can your wifi device see other networks?
Have you tried the speed test on TPG's support page?