Parties do both qualitative ( this just focus groups) and quantitative polling ( this is just like public polls like newspoll). Quite a lot of the time one is good but the other is awful. Way back in 1993 Paul Keating made public with this . The ALP 's polling was terrific on focus groups and which Keating used to perfection but very poor on the polling overall which had it losing bigtime to the Coalition.
In am election campaign both parties do each each weekly.
They NEVER simply poll ONE marginal seat as it costs way too much. They will poll say 4 marginal seats. Take for example 250 for each electorate and then hope the swing is the same in all four seats. The margin of error is far too large in the individual seats to take it seriously.
Both parties will lie their guts out to journalists about their internal polling which will be always at odds with published polls but the journalists never query. We also have quite a few interest groups who will commission polls by the polling agencies in individual seats so in theory it should be harder to do this never underestimate the gullibility of journalists,
The only caveat to this would be if say a multi-millionaire was the leader of a party and was in a very tight election and then spent bigtime to get a better picture but in this case it would be he not the party paying for it.
Just for the gullible, punters do not have any inside information except for the fact they might know people who feel confident.