Can I purchase an additional amount of data before it runs out?
No.
I don't want to my speed to drop down at all if I can avoid it (perhaps an extra 5GB or so would do it)?
Change your plan to get pro rata usage allowance to cover the remainder of the period. Even downgrading to the 50GB plan will do the trick, if you have 5 days remaining in the current bill period as of tomorrow, and request a plan change by self-service in My BigPond before 6PM AEST, since (5 - 1)⁄31 will give you more than 10% of the new plan's included monthly usage allowance to cover the final 4 days.
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Departed from CrowdSupport as of 12 June 2013, when the fun factor for me had finally completely evaporated with all the recent site layout and functionality changes, and the apparent efforts to turn a community goodwill-powered vehicle into something closer to a customer service channel.
The opinions and sentiments expressed above are mine only, and do not necessarily reflect Telstra's views or position. I work at Telstra, but my participation here is strictly in a personal capacity as a fellow Australian telecommunications services consumer, and you can safely assume you are not my customer, client, patron, benefactor or friend when I post in this forum.
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