It turns out to be not so simple. The story of the 1999 Republic referendum polling is one where only the last few months of data are all that usable, and in public polls at least, Yes was never all that much in front.
A summary of Republic polling is available in one paper available online but I thought, surely there was more? I could not find any usable online archive of Republic referendum polling but what I did find, to my surprise, was some polling commentary in an article I published in Togatus April 1999. 1999 model me - not then a polling analyst so please cut some slack for misusing "push-poll" for a seriously bad polling question - wrote this:
"Poll results are confusing. In the last week of January the Age/AC Nielsen showed a national 41% yes vote, but Newspoll showed 58%.[*] The latter was a virtual push-poll because it included the statement 'this will most likely mean that the head of state will not be a politician' and ARM won't get even that simple message through to a thick republican public that mistakes an extra election with people-power."
[* 59% actually. This Newspoll wording was said to have been sourced from the ARM, and was roundly and rightly condemned by opponents as out of step with other polling at the time.]
In fact, question wording was a major issue in 1999 polling, and a case where public polling itself influenced the ballot paper design for the better.