A further 40-60mm is expected between midday and midnight Friday.
"This is the major system that is coming through for the next five days," Mr Coutts said.
"Most of the action is through to early tomorrow morning. There is still more rain to come. Most of the rain should be over before dawn but it is not warning criteria."
After that, Mr Coutts expected the weather to fine up. He said this afternoon the low was still north of Auckland.
"It has got a fair way to come. There is just patchy, light showers over the Bay at the moment."
Mr Coutts said the low was moving across late tomorrow and into Saturday, with south westerlies and fine showers forecast.
He said the Bay had experienced some warm nights with an overnight low of 13 degrees on Wednesday night.
"That is pretty warm and 6-7 degrees above average."
But he said cooler temperatures at night were expected to return.
MetService issued the Bay of Plenty and northern Gisborne with a severe weather warning this morning.
MetService advised people in those areas to look out for rapidly rising rivers and streams. Slips and surface flooding were also possible.
Heavy rain caused flooding and slips in the Western Bay of Plenty during the storms in April.
A Western Bay of Plenty District Council spokeswoman said this afternoon that so far no reported slips had been reported. She said the rain was not at a concerning level.
A Tauranga City Council spokesperson said it was an ordinary rainy day so far.
Standard heavy rain protocols were listed on council's website which encouraged residents to clear any leaves or debris from the gutter.
The base track on Mauao was closed for public safety this afternoon due to large swells.
Wave heights in the Tauranga Harbour reached 6.6 metres.
"Swells of this size can sometimes come up over the track and risk the safety of users," a Tauranga City Council spokesperson said.
All other tracks remain open. Council will reassess the closure tomorrow morning.
HEAVY RAIN EXPECTED:
- 80-100mm of rain could accumulate
- Thunderstorms are possible
- Peak intensities of 15-25mm per hour
Get prepared
Tauranga City Council recommends doing the following to prepare for heavy rain and flooding:
- Keep your roof gutters clean and in good order.
- Check your drainpipes regularly to clear blockages and make sure they work well.
- If your property has flooded in the past, consider keeping a supply of sandbags to prevent future flooding
- If your internal or private drains are overflowing, this is your responsibility and you will need a private drainage contractor to deal with the blockage.