Not the sole issue, but significant
While Turnbull promised a "technology neutral" review of the NBN, that's not what we got. The Coalition policy is still a "cheap as possible, as long as it's faster" policy focused solely on downloads. Labor is doing better, but have been losing out in other respects; their NBN is better conceptually, but they should have been far more open about their screwups.
Reviewing the policies of the three parties in this and other areas:
NBN: Coalition - bad, Labor - Good, Greens - Good
Environment: Coalition - bad, Labor - adequate, Greens - good (although some policies such as their position on nuclear energy I disagree with)
Economic Management: Coalition - poor, Labor - adequate (*), Greens - N/A so far
Equity (giving poorer people a fair go): Coalition - terrible, Labor - OK, Greens - OK. It's broadly true to say that the Coalition favour businesses and supply-side, where Labor favours employees and demand-side.
Human rights: Coalition - terrible, Labor - terrible, Greens - Good. Treatment of refugees has been a national disgrace.
(*) The main reason why the country is having deficit issues is due to structural weaknesses in the tax base introduced under Howard. The country was in a major boom, and any surplus was regarded as a good surplus, so tax cuts (long term revenue reductions) were introduced to trim it down while still retaining some surplus. When the economy went into a downturn, tax receipts dried up leaving us with a deep structural deficit in the federal budget.
The Abbott government is determined to blame all this on Labor (which is why every second sentence from Hockey is about how Labor screwed up).