While many of us were welcoming in 2012, more than 11,000 people decided that New Year's Eve was just the time to file their tax return. A hardcore of 102 diligent taxpayers opted to forsake the pubs, parties and Jools' Annual Hootenanny on the BBC, and submitted their return between 11pm and midnight that evening. A further 8,900 people sent back their online tax return on New Year's Day.

The partygoers among us may see them as a dull lot, but we might change our tune as the self-assessment deadline date of midnight on 31 January approaches. More than half a million people filed their online tax return that day last year.

However, leaving it to the last minute is becoming a dangerous game as HM Revenue & Customs (HMRC) gets tougher powers to deal with laggards. "If you get your return in a minute after midnight on 1 February, you will probably get a penalty notice," says Tina Riches, technical director at the Chartered Institute of Taxation. A new system has come in which no longer provides the 24 hours' grace that saved so many in the past from £100 automatic penalties.

The new system also does away with the old rule that limited the automatic fine for late filing to the amount of tax owing. Fines will be levied "regardless of whether tax is due or not", says an HMRC spokesman. Its website adds: "The longer you delay, the more you'll have to pay. If your tax return is three months late, you'll have to pay a penalty for each additional day it is late. If it's six months late, you'll have to pay a further penalty, and another final penalty if it's 12 months late. Together these could add up to £1,600 or more."

And don't send in a paper tax return now – the deadline was 31 October 2011, and you will have to pay a £100 penalty straight away if you do.

With nearly one million people filing their return late last year, there clearly needs to be a change in awareness and attitudes to avoid widespread fines. Many of last year's late filers may not have owed tax, so avoided a fine. If they do not wake up to the new system, however, this year HMRC could £100m the richer.

These penalties arrive hard on the heels of other reforms that took effect from 2009-10, giving more latitude to HMRC over fining people for incorrect data in their returns. "They are really beginning to bite now," says Riches. "We are seeing more and more penalties for errors."

The 2009-10 reforms allow HMRC to levy penalties of up to 30% of the tax due for "careless" mistakes. Of course, a tax inspector's idea of a careless error could be broader than that of the person he or she is investigating. The penalties rise to 70% of the tax due for "deliberate" mis-statement, and then to 100% for mis-statements that are both deliberate and concealed.

When it comes to late filing, the automatic £100 fine is just the first in a series of penalties. HMRC gives the example of Mrs A, who does not get her return in until 5 August, and so accumulates penalties of £1,300. This is made up of the initial £100 fine, then a £10-a-day fine that begins after three months and lasts for 90 days; then a £300 fine when the return is six months overdue (or 5% of any tax owing, if that produces a higher figure).

Under the tax laws, HMRC must accept a "reasonable excuse", such as serious illness of the person filing, for late returns. But it appears reluctant to do so as this also means waiving the fines. "The Revenue is taking a very narrow interpretation of reasonable excuse," says Stephen Herring, senior tax partner at accountant BDO.

And there is another important reason for making sure you get your tax return in on time. HMRC is taking into account taxpayers' behaviour in terms of tax compliance, to help determine where it will focus its investigations.

Riches says: "If you persistently send your tax return in late or pay tax late, they mark you down as someone perhaps not taking their tax responsibilities so seriously. You are more likely to have your tax return inquired into."

The taxman is also focusing on particular groups. At present the categories whose returns are most likely to be carefully scrutinised are:

• Doctors and dentists: Thousands are suspected of having underpaid tax. HMRC has collected information from locum agencies, drug companies and others that could prove certain individuals have not declared all their earnings. The campaign targeting doctors and dentists has already raised at least £10m. It is understood one doctor who owed the taxman £1m paid the whole sum with a cheque.

• Landlords: People renting out properties abroad or owning three or more in the UK are actively being examined. A common "error" is to include the capital repayment element of a traditional mortgage as an expense. Only the interest is an eligible tax expense.

• Private tutors: HMRC has turned its attention to tax-dodging by private tutors, which could include music and language teachers, horseriding instructors and fitness coaches as well as teachers working as private tutors to supplement their incomes.

An investigation into your tax return is something all taxpayers would want to avoid.

Once HMRC has cause to be suspicious, specialist investigators will comb every item of income and expenditure, seeking proof of even the smallest items and extrapolating any under-declarations they find back into previous years. Many people take professional advice if they become subject to an investigation, and can easily pay four- and five-figure sums for that help.

Herring, of BDO, urges people to get working on their tax returns now. "The key issue we see is people starting the process too late. It's really important they give themselves two to three weeks. Things come up, like your not knowing if the bank interest you received has been paid gross or net of tax."

So maybe last weekend's 20,000 filers were the canny ones. They could buy themselves a small case of bubbly with the £100 they save from not getting that fine.