BAS deadlines: a plain-English calendar

Most businesses lodge their BAS quarterly, and the due dates follow a predictable pattern, with one exception that trips people up every single year. Here's the current calendar (2026–27 financial year), plus the pattern to remember so you're never caught guessing.

The 2026–27 quarterly calendar

(Exact dates shift by a day or two some years when the standard date falls on a weekend — worth a quick double-check against ato.gov.au close to each due date, or just let your BAS agent track it for you.)

The pattern worth remembering

Three of the four quarters follow the same rule: roughly a month after the quarter ends if you lodge it yourself, with an extra few weeks if a registered BAS agent lodges it for you. The exception is the December quarter (Q2),  because it falls over the Christmas and New Year period, the ATO already builds in a longer gap (to 28 February) for everyone, so there's no additional agent extension on top of it that quarter.

Why the agent extension exists and why it's not automatic

The later date for agent-lodged BAS isn't a loophole, it's built into the system because agents are lodging on behalf of many clients at once. But it only applies once you're actually set up with a registered agent lodging through their systems; it's not something you get just by asking for more time. If cash flow around a due date is tight, that's worth raising well before the deadline, not on the day.

What happens if you miss one

A late BAS can attract a Failure to Lodge penalty, calculated in penalty units for each 28-day period it's overdue (capped after a certain point). The ATO can also apply general interest charge on any amount owing. Neither is usually catastrophic for a single quarter lodged a few days late, especially if you get in touch, but it compounds if it becomes a pattern, so it's worth flagging early if a due date is going to be missed rather than letting it pass quietly.